Peroxiredoxin VI oxidation in cerebrospinal fluid correlates with traumatic brain injury outcome

Traumatic brain injury (TBI) patients would benefit from the identification of reliable biomarkers to predict outcomes and treatment strategies. In our study, cerebrospinal fluid (CSF) from patients with severe TBI was evaluated for oxidant stress-mediated damage progression after hospital admission and subsequent ventriculostomy placement. Interestingly, substantial levels of peroxiredoxin VI (Prdx6), a major antioxidant enzyme normally found in astrocytes, were detected in CSF from control and TBI patients and were not associated with blood contamination. Functionally, Prdx6 and its associated binding partner glutathione S-transferase Pi (GSTP1-1, also detected in CSF) act in tandem to detoxify lipid peroxidation damage to membranes. We found Prdx6 was fully active in CSF of control patients but becomes significantly inactivated (oxidized) in TBI. Furthermore, significant and progressive oxidation of "buried" protein thiols in CSF of TBI patients (compared to those of nontrauma controls) was detected over a 24-h period after hospital admission, with increased oxidation correlating with severity of trauma. Conversely, recovery of Prdx6 activity after 24h indicated more favorable patient outcome. Not only is this the first report of an extracellular form of Prdx6 but also the first report of its detection at a substantial level in CSF. Taken together, our data suggest a meaningful correlation between TBI-initiated oxidation of Prdx6, its specific phospholipid hydroperoxide peroxidase activity, and severity of trauma outcome. Consequently, we propose that Prdx6 redox status detection has the potential to be a biomarker for TBI outcome and a future indicator of therapeutic efficacy.

Inhibition of thromboxane synthase activity modulates bladder cancer cell responses to chemotherapeutic agents

Recently, we reported prognostic significance of thromboxane synthase (TXAS) gene expression in invasive bladder cancer. The positive correlation between elevated TXAS expression and shorter patient survival supports a potential role for TXAS-regulated pathways in tumor metastases. In this study, using immunohistochemical analysis, we found an increased expression of TXAS protein in bladder cancer. Treatment of T24 and transitional cell carcinoma TCC-SUP bladder cancer cells with the TXAS inhibitors furegrelate or ozagrel induced an apoptotic effect measured as an increase in caspase-3 activation and cell death, and decreased survivin expression. Pharmacological inhibition of TXAS using the TXAS inhibitor furegrelate increased sensitivity to the chemotherapeutic agents cisplatin and paclitaxel. Molecular inhibition of TXAS expression by siRNA significantly decreased cell growth and migration. In concordance with the pharmacological data, siRNA-mediated reduction of TXAS expression increased sensitivity to cisplatin and paclitaxel in T24 and TCC-SUP cells. In summary, the data support a role for the thromboxane A(2) pathway in the pathogenesis of bladder cancer and the potential utility of modulation of this signaling pathway for cancer chemotherapy.

Castration reduces platelet thromboxane A2 receptor density and aggregability

BACKGROUND

Exogenously administered testosterone upregulates platelet thromboxane A2 (TXA2) receptors and increases aggregation response to thromboxane mimetics in healthy male volunteers. However, the biological impact of endogenous testosterone on platelet TXA2 receptor expression, especially in older men at risk of coronary artery disease, is unclear.

AIM

To investigate the impact of reduction in circulating testosterone on platelet TXA2 receptor expression in older men.

DESIGN

Cross-sectional case-control study.

METHODS

We studied surgically and/or medically castrated men with prostate cancer (group A, n = 8, aged 71 +/- 8 years) and age-matched, uncastrated urology patients (group B, n = 7, aged 67 +/- 9 years). Plasma testosterone was measured by radioimmunoassay. Platelet TXA2 receptor expression was assessed by radioligand binding studies using radioactive 125I-BOP. Platelet aggregation responses to TXA2-mimetic I-BOP, and to thrombin, were also studied.

RESULTS

Group A had significantly lower plasma testosterone than group B (16 +/- 5 ng/dl vs. 308 +/- 47 ng/dl, p<0.001). Platelet TXA2 receptor density (B(max)) but not affinity (K(d)) was lower in group A (0.50 +/- 0.12 vs. 1.01 +/- 0.17 pmol/mg protein, p = 0.03). Maximum platelet aggregation response to I-BOP (E(max)), but not sensitivity (EC50) was lower in group A (53 +/- 2% vs. 63 +/- 2%, p = 0.003 ANOVA). In vitro, high concentrations of hydroxyflutamide (100 microM) competitively inhibited U46619-induced platelet aggregation in washed platelets, without affecting the binding of 125I-BOP to platelet TXA2 receptors.

DISCUSSION

Endogenous testosterone regulates platelet TXA2 receptor B(max) and the E(max) aggregation response to thromboxane mimetic I-BOP. Blockade of androgen receptors or inhibition of testosterone production may reduce platelet aggregation responses. Preliminary evidence suggests the presence of functional androgen receptors on human platelets, which may regulate TXA2 receptor expression.

Changes in vascular responsiveness to a thromboxane mimetic in endotoxin tolerance

Our previous studies have demonstrated that peritoneal macrophages obtained from endotoxin-tolerant rats exhibit altered cellular activation by endotoxin, possibly involving changes in guanine nucleotide regulatory (G) protein-coupled signal transduction pathways. Endotoxin-tolerant rats also exhibit cross tolerance and altered hemodynamic responses to thromboxane (Tx)A2 mimetics, suggesting potential changes in vascular responsiveness. We tested the hypothesis that endotoxin tolerance results in vascular hyporesponsiveness to a TxA2 mimetic via alterations in the TxA2 receptor, G protein function, and/or second messenger production. Rats were rendered endotoxin tolerant by increasing sublethal consecutive doses of Salmonella enteritidis endotoxin (100 to 5000 micrograms/kg, i.p.) for 4 days. The animals were sacrificed 2 days after the final dose of endotoxin for removal of aortas. Contractile responses of aortic rings to U46619, a TxA2 agonist, were assessed in control and tolerant rats. The EC50 values for U46619 were 14.8 +/- 6.6 nM and 32.3 +/- 3.1 nM (n = 5-7), (P < 0.05) for control and tolerant rats, respectively. Crude membranes were prepared from aortas of control and tolerant rats, and binding of I-BOP TxA2/PGH2 receptor agonist, [1S-(1 alpha, 2 beta (5Z), 3 alpha (1E, 3S*), 4 alpha)]-7-[3-(3-hydroxy-4-(4'-iodophenoxy)-1-butenyl)-7- oxabicyclo-[2.2.1]heptan-2-yl]-5-heptenoic acid (I-BOP), a TxA2 agonist, was assessed by Scatchard analysis. I-BOP binding to the TxA2 receptor was saturable and revealed a single class of TxA2 receptors for both groups. There was no significant difference in control (n = 7) compared with tolerant (n = 5) Kd values (2.1 +/- 0.2 vs. 2.4 +/- 0.9 nM, respectively), or Bmax (31 +/- 6 vs. 28 +/- 12 fmol/mg protein, respectively). To assess potential changes in G protein function, aortic membrane GTpase activity was determined. GTPase activity in tolerant membranes was significantly reduced (P < 0.05) compared with control membranes (309 +/- 23 (n = 5) vs. 440 +/- 32 (n = 7) pmol/mg/protein/min, respectively). However, U46619-stimulated phosphoinositide production was similar in vascular tissue from control and tolerant rats. These observations suggest that the decreased contractile response to TxA2 mimetics in endotoxin tolerance does not result from a change in receptor number, affinity of TxA2 receptors, or changes in phosphatidylinositol metabolism but is associated with decreased vascular G protein function.

Effect of cross-tolerance between endotoxin and TNF-alpha or IL-1beta on cellular signaling and mediator production

Endotoxin [lipopolysaccharide (LPS)] tolerance suppresses macrophage/monocyte proinflammatory-mediator production. This phenomenon also confers cross-tolerance to other stimuli including tumor necrosis factor (TNF) alpha and interleukin (IL)-1beta. Post-receptor convergence of signal transduction pathways might occur after LPS, IL-1beta, and TNF-alpha stimulation. Therefore, it was hypothesized that down-regulation of common signaling molecules induces cross-tolerance among these stimuli. LPS tolerance and cross-tolerance were examined in THP-1 cells. Phosphorylation of MAP kinases and degradation of inhibitor kappaBalpha (IkappaBalpha) DNA binding of nuclear factor-kappaB (NF-kappaB), and mediator production were examined. In naive cells, LPS, TNF-alpha, and IL-1beta induced IkappaBalpha degradation, kinase phosphorylation, and NF-kappaB DNA binding. LPS stimulation induced production of TNF-alpha or TxB2 and degradation of IRAK. However, neither TNF-alpha nor IL-1beta induced IRAK degradation or stimulated TNF-alpha or TxB2 production in naive cells. Pretreatment with each stimulus induced homologous tolerance to restimulation with the same agonist. LPS tolerance also suppressed LPS-induced TxB2 and TNF-alpha production. LPS pretreatment induced cross-tolerance to TNF-alpha or IL-1beta stimulation. Pretreatment with TNF-alpha induced cross-tolerance to LPS-induced signaling events and TxB2 production. Although pretreatment with IL-1beta did not induce cross-tolerance to LPS-induced signaling events, it strongly inhibited LPS TNF-alpha and TxB2 production. These data demonstrate that IL-1beta induces cross-tolerance to LPS-induced mediator production without suppressing LPS-induced signaling to MAP kinases or NF-kappaB activation.

Carbon dioxide is the major metabolite of quercetin in humans

A previous study in ileostomy patients indicated that dietary glucosides of the flavonoid quercetin are hydrolyzed efficiently in the intestinal lumen, followed by absorption of a large fraction of the quercetin aglycone. To determine the fate of quercetin, we administered 1.85 MBq (50 microCi) of (14)C-quercetin both orally (100 mg, 330 micromol) and intravenously (iv; 0.3 mg, 1 micromol) to healthy volunteers. Serial plasma samples, urines and stools were collected for 72 h. Total radioactivity was determined by liquid scintillation spectrometry directly in plasma and urine and after repeated methanol extraction of stool homogenate samples. The oral absorption, based on total radioactivity, was surprisingly high, ranging from 36.4 to 53.0%. The biological half-life was very long, ranging from 20 to 72 h. The urinary recovery of total radioactivity ranged from 18.4 to 26.8% after the iv dose and from 3.3 to 5.7% after the oral dose. The corresponding fecal recoveries were only 1.5-5.0% and 1.6-4.6%, respectively. Thus, the total recovery of the (14)C-quercetin doses, in particular after oral administration, was very low. In search for the unaccounted for fraction of the (14)C-quercetin dose, we performed (14)CO(2) recovery studies in three volunteers (3 iv and 3 oral doses). At timed intervals, (14)CO(2) in expired air was trapped in hyamine hydroxide/thymolphthalein and analyzed for radioactivity. As much as 23.0-81.1% of the quercetin dose was recovered as (14)CO(2) in the expired air from these volunteers, after both oral and iv doses. The disposition of quercetin in humans is thus highly complex, requiring further studies.

Progesterone regulation of vascular thromboxane A(2) receptors in rhesus monkeys

We hypothesized that progesterone regulates thromboxane A(2) receptor (TxA(2)R) density in primate vascular muscle and that TxA(2)R density correlates with coronary reactivity in vivo and in vitro. Reactivity to serotonin + U-46619 was determined by angiography in surgically postmenopausal [ovariectomized (Ovx)] rhesus monkeys without progesterone replacement and after 2-wk progesterone treatment (1-2 ng/ml). In untreated Ovx animals, 100 micromol/l serotonin + 1 micromol/l U-46619 (syringe concentrations) provoked vasospasm-like constrictions in six of six monkeys; zero of six progesterone-treated monkeys developed vasospasms. Sustained Ca(2+) responses in vascular muscle cells isolated from Ovx coronaries (208 +/- 63% of basal 20 min after stimulation) treated with serotonin + U-46619 contrasted with transient Ca(2+) responses (143 +/- 18% of basal and decreasing 5 min after stimulation) in progesterone-treated monkeys. The maximum density of [1S-(1I,2J(5Z),3I(1E,3R*),4I)]-7-[3-(3-hydroxy-4-(4'-[(125)I]iodophenoxy)- 1-butenyl)-7-oxabicyclo[2.2.1]heptan-2-yl]-5-heptenoic acid ([(125)I]-BOP) binding was greater (P < 0.01) in carotid arteries and aortic membranes from Ovx (109 +/- 11 fmol/mg) compared with progesterone-treated (43 +/- 15 fmol/mg) monkeys. TxA(2)R immunolabeling revealed greater coronary TxA(2)R labeling in Ovx compared with progesterone-treated monkeys. The results suggest that progesterone can decrease arterial TxA(2)R in Ovx monkeys.

Signal transduction events in Chinese hamster ovary cells expressing human CD14; effect of endotoxin desensitization

Previous studies suggest that endotoxin (LPS) stimulation of CD14 receptors may be coupled to heterotrimeric G proteins. However, characterization of the G protein-coupled signaling pathways is incomplete. Also, specific changes in the transduction pathways occur in a phenomenon known as LPS tolerance or desensitization induced by prior exposure to LPS. In the present study, we examined potential CD14-dependent G protein-coupled signaling events in response to LPS, and changes in signaling in these pathways during LPS desensitization in Chinese Hamster Ovary (CHO) cells. LPS stimulated inhibitory kappa B alpha (IkappaB alpha) degradation and p38 phosphorylation in CHO cells transfected with human CD14 receptor (CHO-CD14), but not in CHO cells transfected with vector only. However, activation of these signaling events diverged early in the signal transduction pathways. Pretreatment with pertussis toxin, which inactivates inhibitor G protein (G alpha i) function, significantly inhibited LPS-induced p38 phosphorylation, but not LPS-induced IkappaB alpha degradation. Mastoparan, a putative G alpha i agonist, synergized with LPS to induce p38 phosphorylation. Thus, LPS stimulation of p38 phosphorylation is, in part, G alpha i coupled, whereas IkappaB alpha degradation is not. In subsequent studies, CHO-CD14 cells were desensitized by prior LPS exposure. LPS-desensitized cells exhibited augmented IkappaB alpha content and were refractory to LPS-induced IkappaB alpha degradation and p38 phosphorylation. Pretreatment with cycloheximide, a protein synthesis inhibitor, prevented the effect of LPS desensitization on augmenting cellular IkappaB alpha content and its refractoriness to LPS-induced degradation. However, cycloheximide pretreatment did not prevent impaired p38 phosphorylation in desensitized cells. IkappaB alpha upregulation in LPS tolerance may occur through increased synthesis and/or induction of protein that suppress IkappaB alpha degradation. The latter protein synthesis-dependent mechanisms may be distinct from mechanismis inhibiting p38 phosphorylation in tolerance. These findings suggest that LPS tolerance induces CD14-dependent signaling alterations in G alpha i-coupled pathways leading to mitogen-activated (MAP) kinase activation as well as G alpha i-independent pathways inducing IkappaB alpha degradation.

Prostaglandin J(2) inhibition of mesangial cell iNOS expression

Mesangial cells from MRL/lpr mice, a model of lupus, overproduce nitric oxide (NO) compared to controls. J series prostaglandins (PG) and thiazolidinediones block LPS stimulation of NO production via the activation of peroxisome proliferator-activator receptor-gamma (PPAR-gamma) in macrophages but utilize an alternative mechanism in microglial cells. We investigated the mechanism by which PGJ(2) inhibits NO production in LPS/IFN-gamma-stimulated MRL/lpr mesangial cells. Our results demonstrated that LPS/IFN-gamma addition to MRL/lpr mesangial cells stimulated iNOS activation, expression of p-38 kinase and p44/42 MAPK, and NF-kappaB translocation to the nucleus. Both pioglitazone, a specific PPAR-gamma agonist, and PGJ(2) blocked NO production, iNOS protein expression, and iNOS mRNA transcription. PGJ(2) failed to inhibit nuclear NF-kappaB translocation or p44/42 MAPK or p-38 kinase induction in stimulated mesangial cells. These data suggest that PGJ(2) blocks iNOS expression and subsequent NO production in mesangial cells via a PPAR-gamma-mediated mechanism either by interfering with NF-kappaB transcriptional activity or by an NF-kappaB-independent mechanism.

Disposition and metabolism of the flavonoid chrysin in normal volunteers

AIMS

To describe the oral disposition of the dietary flavonoid chrysin in healthy volunteers.

METHODS

Oral 400 mg doses of chrysin were administered to seven subjects. Chrysin and metabolites were assayed in plasma, urine and faeces by h.p.l.c.

RESULTS

Peak plasma chrysin concentrations were only 3-16 ng ml(-1) with AUCs of 5-193 ng ml(-1) h. Plasma chrysin sulphate concentrations were 30-fold higher (AUC 450-4220 ng ml(-1) h). In urine, chrysin and chrysin glucuronide accounted for 0.2-3.1 mg and 2-26 mg, respectively. Most of the dose appeared in faeces as chrysin. Parallel experiments in rats showed high bile concentrations of chrysin conjugates.

CONCLUSIONS

These findings, together with previous data using Caco-2 cells, suggest that chrysin has low oral bioavailability, mainly due to extensive metabolism and efflux of metabolites back into the intestine for hydrolysis and faecal elimination.

Suppression of Cox-2 and TNF-alpha mRNA in endotoxin tolerance: effect of cycloheximide, antinomycin D, and okadaic acid

Lipopolysaccharide (LPS)-tolerant human promonocytic THP-1 cells produce decreased levels of inflammatory mediators such as eicosanoids and tumor necrosis factor alpha (TNFalpha) in response to LPS. We hypothesized that transcriptional repression by newly synthesized proteins may be a mechanism for the reduced cellular response to a secondary challenge with LPS. THP-1 cells were desensitized after a 3.5 h or 20 h pre-exposure to LPS (1 microg/mL) and subsequently challenged with LPS (10 microg/mL). In cells rendered tolerant by exposure to LPS for 20 h, LPS-induced expression of cyclooxygenase (Cox)-2 and TNFalpha mRNA was suppressed. Cycloheximide (10 microM) prevented the transcriptional down-regulation of Cox-2 mRNA and to a lesser extent, TNFalpha mRNA, in LPS-tolerant cells. Transcriptional arrest with actinomycin D stabilized steady-state expression of Cox-2 mRNA in naive and tolerant cells but destabilized TNFalpha mRNA expression in LPS-tolerant cells. The observation that in naive cells Cox-2 and TNFalpha mRNA levels subside at 3 to 4 h after LPS (10 microg/mL or 1 microg/mL) suggested that LPS tolerance may occur earlier. Therefore, in subsequent experiments, the effect of LPS pretreatment for only 3.5 h was examined. This abbreviated tolerance regimen diminished secondary LPS-induced Cox-2 mRNA expression but had a lesser effect on TNFalpha mRNA expression. However, cycloheximide augmented both Cox-2 and TNFalpha mRNA expression in this group. Also, the serine/threonine phosphatase inhibitor okadaic acid augmented Cox-2 and TNFalpha mRNA expression in the LPS-tolerant cells. Although LPS-induced TNFalpha production in LPS-tolerant cells was suppressed relative to the naive cells, okadaic acid induced comparable levels of TNFalpha in tolerant and naive cells. These findings support the concept that LPS tolerance is associated with induction of proteins that alter expression of certain genes. Expression of Cox-2 mRNA appears to be particularly sensitive to down-regulation and, to a lesser extent, TNFalpha mRNA. However, this seems to vary depending on the LPS pretreatment regimen. The ability of a phosphatase inhibitor to induce TNFalpha and expression of Cox-2 and TNFalpha mRNA in LPS tolerance suggests that there may be alterations in phosphorylation status of signaling pathways, transcriptional mechanisms, or post-transcriptional mRNA stability.

Inhibition of mesangial cell nitric oxide in MRL/lpr mice by prostaglandin J2 and proliferator activation receptor-gamma agonists

MRL/Mp-lpr/lpr (MRL/lpr) mice develop immune complex glomerulonephritis similar to human lupus. Glomerular mesangial cells are key modulators of the inflammatory response in lupus nephritis. When activated, these cells secrete inflammatory mediators including NO and products of cyclooxygenase perpetuating the local inflammatory response. PGJ2, a product of cyclooxygenase, is a potent in vitro inhibitor of macrophage inflammatory functions and is postulated to function as an in vivo inhibitor of macrophage-mediated inflammatory responses. We hypothesized that in lupus, a defect in PGJ2 production allows the inflammatory response to continue unchecked. To test this hypothesis, mesangial cells were isolated from MRL/lpr and BALB/c mice and stimulated with IL-1beta or LPS plus IFN-gamma. In contrast to the 2- to 3-fold increase in PGJ2 production by stimulated BALB/c mesangial cells, supernatant PGJ2 did not increase in MRL/lpr mesangial cell cultures. NO production in stimulated MRL/lpr and BALB/c mesangial cells, was blocked by PGJ2 and pioglitazone. These studies suggest that abnormalities in PGJ2 production are present in MRL/lpr mice and may be linked to the heightened activation state of mesangial cells in these mice.

Coupling of thromboxane A2 receptor isoforms to Galpha13: effects on ligand binding and signalling

Previous subtyping of thromboxane A2 (TXA2) receptors in platelets and vascular smooth muscle cells was based on pharmacological criteria. Two distinct carboxy-terminal splice variants for TXA2 receptors exist and they couple to several different G protein alpha subunits including Galpha13, but it has not been established whether either or both isoforms interact with and signal through it. We sought to determine: (1) which TXA2 receptor isoforms exist in vascular smooth muscle, (2) if Galpha13 is present in vascular smooth muscle and (3) if Galpha13 interacts with either or both of the two TXA2 receptor isoforms as determined by changes in ligand binding properties and generation of intracellular signals. Both TXA2 receptor isoforms and Galpha13 were found in vascular smooth muscle cells. Both the alpha and beta isoforms of the TXA2 receptors were transiently transfected with or without Galpha13 into COS-7 (radioligand binding assays) or CHO cells (agonist induced Na+/H+ exchange). Co-expression of each receptor isoform with Galpha13 significantly (P<0.05) increased the affinity of each receptor for the two agonists, I-BOP and ONO11113, and decreased the affinity of the receptor for the antagonists, SQ29,548 and L657,925. I-BOP stimulated Na+/H+ exchange in vascular smooth muscle cells. Co-expression of Galpha13 with each TXA2 receptor isoform in CHO cells resulted in a significant (P<0.04) agonist induced increase in Na+/H+ exchange compared to cells not transfected with Galpha13. The results support the possibility that the previous classification of TXA2 receptor subtypes based on pharmacological criteria reflect unique interactions with specific G protein alpha subunits.

Tolerance to LPS decreases macrophage G protein content

The effects of tolerizing doses of LPS on mRNA and protein levels of three different G protein subunits were investigated to understand the mechanism(s) responsible for the reduction in Gialpha protein content in LPS tolerance. Tolerance was induced in rats using Salmonella enteritidis LPS (intraperitoneal route) with a single dose of 100 microg/kg. Peritoneal macrophages were harvested 6 and 24 h later. In some studies, a second dose of LPS 500 microg/kg was given on the following day, and peritoneal macrophages were harvested 5 days after the first injection. Macrophage RNA or a crude membrane fraction was prepared from macrophages, and the mRNA level or the protein content for Gialpha3, Gialpha2, and Gsalpha was analyzed using Northern or Western blots, respectively. Compared with the control levels, the message for Gialpha3 was reduced (p < .025) at 6 and 24 h and 5 day time periods after LPS treatment. The Gialpha2 mRNA was increased relative to the control levels (p < .05) at 6 h and 5 days after LPS treatment, respectively, and Gsalpha message was not significantly changed. The half-life of Gialpha3 mRNA was not significantly different in control versus tolerant macrophages. The Gialpha3 mRNA and membrane protein were not significantly changed by incubation with LPS for intervals up to 6 h in vitro. Macrophage membrane Gialpha3 and Gialpha1 and 2 protein content from tolerant rats were reduced compared with the controls at 6 and 24 h, respectively (p < .05). These studies are consistent with our previous observations of selective changes in macrophage Gialpha protein content in LPS tolerance and raise the possibility that this may affect signal transduction events in these cells.

Expression, characterization, and purification of C-terminally hexahistidine-tagged thromboxane A2 receptors

Thromboxane A2 (TxA2) receptors belong to the class of G-protein-coupled receptors. Knowledge of the relationship of structure to function for TxA2 receptors is limited because of their low levels of expression, lengthy purification procedures and poor recoveries. A C-terminal hexahistidine-tag (C-His) was ligated to the alpha-isoform of TxA2 receptors and expressed in COS-7 and Chinese hamster ovary cells. The C-His-TxA2 receptors bound the radioligands 125I-7-[(1R,2S,3S,5R)-6, 6-dimethyl-3-(4-benzenesulfonylamino)bicyclo[3.1. 1]hept-2-yl]-5(Z)-heptenoic acid, an antagonist, and 125I-[1S-1alpha, 2beta(5Z),3alpha(1E,3S*), 4alpha]-7-[3[(3-hydroxy-4-(4'-phenoxy)-1butenyl)-7-oxabicycl o-[2.2. 1]heptan-2-yl]-5-heptanoic acid, an agonist, with affinities not significantly different from those of the wild type (wt)-TxA2 receptors. LipofectAMINE transfection of the cDNAs resulted in high levels of expression (Bmax = 95 +/- 6 pmol/mg) of the C-His-TxA2 receptors. In competition binding studies the IC50 values of five different ligands were not significantly different between C-His-TxA2 and wt-TxA2 receptors. Agonist-induced stimulation of cAMP and total inositol phosphate formation were not significantly different between the two receptors. Purification on a Ni2+-NTA column resulted in a rapid (within 4 h) purification with a 36 +/- 2% recovery and a 30 +/- 6-fold purification (n = 5). The partially purified receptors were resolved on SDS-polyacrylamide gel electrophoresis, transferred to a nitrocellulose membrane, dissolved in acetone/trifluoroacetic acid/hexafluoroisopropanol/sinapinic acid, and successfully subjected to matrix-assisted laser desorption ionization-time of flight mass spectrometry analysis. The results suggest that the combination of a high level of expression of C-His-TxA2 receptors and a rapid purification procedure followed by SDS- polyacrylamide gel electrophoresis may provide a useful approach for mass-spectrometry based structure-function and other studies of TxA2 receptors.

Cloning and characterization of an endogenous COS-7 cell thromboxane A2 receptor

A cDNA for a thromboxane A2 (TXA2) receptor was cloned from an SV40 transformed African Green Monkey kidney cell line (COS-7). The sequence is 98% homologous with the isoform of the human TXA2 receptor and has agonist and antagonist ligand binding characteristics that are not significantly different from the human receptor. Stimulation of the COS-7 cells with the TXA2 receptor agonist, ONO 11113 resulted in a significant increase in cAMP formation that was blocked by a receptor antagonist. The results raise the question of the utility of the COS-7 cell line for studies of cloned and expressed TXA2 receptor signalling mechanisms.

Endotoxin tolerance alters macrophage membrane regulatory G proteins

Administration of sublethal doses of endotoxin (LPS) or tumor necrosis factor-alpha (TNF alpha) renders rats tolerant to supralethal doses of LPS. Peritoneal macrophages from tolerant rats are refractory to LPS induced arachidonic acid (AA) metabolism and cytokine production in vivo, and exhibit reduced membrane GTPase activity and GTP gamma S binding. Since LPS stimulated AA metabolism is mediated by Gi alpha proteins, we sought to determine whether Gi alpha and/or other G proteins are reduced in LPS tolerance. Rats were rendered tolerant by two daily sublethal doses of Salmonella enteritidis LPS, 100 micrograms/kg and 500 micrograms/kg administered intraperitoneally. Animals were allowed to rest for 72 hours. Alternatively, tolerance to LPS was induced by sublethal administration of human recombinant TNF alpha (10 micrograms/kg) intraperitoneally 24 hrs before the experiments. Macrophage membrane G protein content was determined by immunoblot analysis with specific antisera to Gi1,2 alpha, Gi3 alpha, Gs alpha and the G protein beta subunits (G beta). Membrane G proteins were differentially decreased in tolerant macrophages. In macrophages from rats rendered tolerant by sublethal doses of LPS, Gi3 alpha was reduced the most to 48 +/- 8% of control (n = 3, P < 0.05) and this reduction was significant compared to those of other G proteins. Gi1,2 alpha and G beta were reduced to 73 +/- 5% (n = 3, P < 0.05) and 65 +/- 4% (n = 3, P < 0.05) of control respectively. Gs alpha(L) and Gs alpha(H) were also reduced to 61 +/- 5% (n = 3, P < 0.05) and 68 +/- 3% (n = 3, P < 0.05) of control, respectively. In contrast, only Gi3 alpha was reduced in macrophage membranes from rats pretreated with TNF alpha. Gi3 alpha was reduced to 57 +/- 11% of control (n = 4, P < 0.05) whereas Gi1,2 alpha and G beta were not significantly affected. These results demonstrate selective changes in tolerant macrophage membrane G proteins and suggest a potential role for Gi3 alpha in mediating LPS tolerance. The molecular mechanisms underlying these changes and their significance in LPS tolerance merit further investigation.

Increased prostacyclin and PGE2 stimulated cAMP production by macrophages from endotoxin-tolerant rats

Sublethal administration of lipopolysaccharide (LPS) renders rats tolerant to multiple lethal stimuli. Tolerant macrophages exhibit differential alterations in LPS-stimulated cytokine and inflammatory mediator release. Increased cAMP levels stimulated by PGE2 or prostacyclin (PGI2) result in differential effects on LPS-induced cytokine release and protect against the pathophysiological changes of endotoxemia. In the present studies, we sought to determine whether PGE2- and PGI2-stimulated cAMP levels are altered in tolerant macrophages. Incubation of macrophages with cicaprost or 11-deoxy-PGE1 in the presence of phosphodiesterase inhibitors resulted in significantly higher (2.5- to 6.5-fold) cAMP concentrations in tolerant macrophages compared with control. In contrast, isoproterenol-stimulated cAMP levels were not significantly different between control and tolerant cells. Also, incubation of tolerant macrophages with LPS did not result in significantly elevated cAMP levels. Prostacyclin (IP) receptor mRNA levels were significantly increased in tolerant cells compared with controls, whereas [3H]PGE2 binding and PGE2 EP4 receptor mRNA levels were not significantly changed. These studies suggest that LPS tolerance induces selective alterations in eicosanoid regulation of cAMP formation.

Mechanism of platelet inhibition by nitric oxide: in vivo phosphorylation of thromboxane receptor by cyclic GMP-dependent protein kinase

Nitric oxide (NO) is a potent vasodilator and inhibitor of platelet activation. NO stimulates production of cGMP and activates cGMP-dependent protein kinase (G kinase), which by an unknown mechanism leads to inhibition of Galphaq-phospholipase C-inositol 1, 4,5-triphosphate signaling and intracellular calcium mobilization for several important agonists, including thromboxane A2 (TXA2). To explore the mechanism of platelet inhibition by NO, activation of platelet TXA2 receptors in the presence of cGMP was studied. The nonhydrolyzable analog 8-bromo-cyclic GMP (8-Br-cGMP) potently inhibited activation of the TXA2-specific GTPase in platelet membranes in a concentration-dependent fashion, suggesting that G kinase catalyzes the phosphorylation of some proximal component of the receptor-G protein signaling pathway. Nanomolar concentrations of G kinase were found to catalyze the phosphorylation of platelet TXA2 receptors in vitro, but not Galphaq copurifying with the TXA2 receptors in these experiments. Using immunoaffinity methods, in vivo phosphorylation of TXA2 receptors by cyclic GMP was demonstrated from 32P-labeled cells treated with 8-Br-cGMP. Peptide mapping studies of in vivo phosphorylated TXA2 receptors demonstrated cGMP mediates phosphorylation of the carboxyl terminus of the TXA2 receptor. G kinase also catalyzed the phosphorylation of peptides corresponding to the cytoplasmic tails of both alpha and beta forms of the receptor but not control peptide or a peptide corresponding to the third intracytoplasmic loop of the TXA2 receptor. These data identify TXA2 receptors as cGMP-dependent protein kinase substrates and support a novel mechanism for the inhibition of cell function by NO in which activation of G kinase inhibits signaling by G protein-coupled receptors by catalyzing their phosphorylation.

Gender-related differences in androgen regulation of thromboxane A2 receptors in rat aortic smooth-muscle cells

Thromboxane A2 (TXA2) has been implicated as an important mediator of cardiovascular diseases. Aortas obtained from male rats are more sensitive to TXA2 mimetics compared with those obtained from females. A similar phenomenon has been reported in canine coronary arteries. To determine whether there is a gender-related difference in the regulation of TXA2 receptors by androgenic steroids, we determined the effect of testosterone and dihydrotestosterone (DHT) on TXA2 receptor density in cultured rat aortic smooth-muscle (RASM) cells and guinea pig coronary artery smooth-muscle (CASM) cells. TXA2 receptor density (B(max)) and dissociation constant (Kd) were determined by radioligand binding studies with (125)I-BOP, a TXA2 receptor agonist. Testosterone significantly (p < 0.05) increased TXA2 receptor density in cultured RASM cells and guinea pig CASM cells. DHT significantly (p < 0.005) increased the B(max) in male RASM cells (62 +/- 2 vs. 40 +/- 3 fmol/mg protein; n = 7; p < 0.005). DHT increased the B(max) values in both male and female RASM cells, but the increase was significantly (p < 0.05) less in female than in male RASM cells (57 +/- 10% increase for male and 31 +/- 5% for female). Androgen-receptor protein was detected in RASM cells by Western blot and was less in the female RASM cells than in the male. The results indicate that RASM cells possess an androgen receptor and that gender-related differences exist in the regulation of expression of TXA2 receptors by androgens.

Thromboxane A2 receptor density increases during chronic exposure to thromboxane A2 receptor antagonists after porcine carotid bypass

Domestic swine (n=12 in each group) were randomized to daily treatment with the thromboxane A2 (TXA2) receptor antagonist BMS-180291 (group I), aspirin (group II), or no drug (group III) prior to prosthetic carotid graft implantation. Platelet and arterial wall receptor density were measured by equilibrium binding using 125I-BOP. At 6 weeks, means (s.e.m.) platelet receptor density (pmol/mg) had increased in groups I (3.3(0.6) versus 1.8(0.3); P<0.05) and II (2.6(0.6) versus 1.7(0.2); P<0.05), but not in group III (1.3(0.3) versus 1.2(0.2)). Aortic membrane TXA2 receptor density (fmol/mg) was significantly greater (P<0.05) in groups I (150(50)) and II (68(10)) compared with group III (39(6)). Chronic exposure to a TXA2 receptor antagonist or aspirin is associated with increased platelet and aortic receptor density in pigs.

Inhibition of testosterone 5 alpha-reductase: evidence for tissue-specific regulation of thromboxane A2 receptors

Testosterone has been implicated as a risk factor for cardiovascular diseases and thromboxane A2 (TXA2) may be an important pathophysiologic mediator for them. Testosterone has been shown to increase TXA2 receptor density in several cell types. Testosterone is reduced at the 5 alpha position to its active metabolite, dihydrotestosterone, by 5 alpha-reductase. We determined the effects of epristeride, a 5 alpha-reductase inhibitor, on the density of TXA2 receptors in rat aortic smooth muscle cells and human erythroleukemia cells, a megakaryocyte-like cell, in vitro, and in rat platelets and aortic membranes in vivo. In rat aortic smooth muscle cells, epristeride significantly (P < .01, n = 5) blocked the effect of testosterone to increase TXA2 receptor density (Bmax: 44 +/- 3, 76 +/- 7, 48 +/- 4 and 46 +/- 4 fmol/mg protein, for control cells, cells treated with testosterone (200 nM), cells treated with testosterone and epristeride (10 nM) and cells treated with epristeride, respectively. Epristeride did not block the effect of testosterone in human erythroleukemia cells. Treatment of male rats with epristeride for 2 weeks significantly (P < .01) decreased TXA2 receptor density in aortic membranes (41 +/- 3 for vehicle, n = 10; 27 +/- 3 fmol/mg protein for epristeride, n = 11) but did not significantly change TXA2 receptor density in platelets. Maximum contractile responses of rat aortas to U46619, a TXA2 mimetic, were significantly (P < .001) lower in epristeride-treated rats than in vehicle-treated rats (4.2 +/- 0.1 for vehicle, n = 16, 3.0 +/- 0.2 g tension for epristeride, n = 15). In conclusion, regulation of expression of TXA2 receptors by testosterone in cells of vascular origin, but not in platelets, appears to be via DHT.

Alterations in macrophage G proteins are associated with endotoxin tolerance

Previous studies have suggested that endotoxin tolerance induces macrophage desensitization to endotoxin through altered guanine nucleotide regulatory (G) protein function. In the present study the binding characteristics of the nonhydrolyzable GTP analogue GTP gamma [35S] to macrophage membranes from endotoxin tolerant and control rats were determined. Membranes were prepared from peritoneal macrophages harvested from rats 72 h after two sequential daily doses of vehicle or Salmonella enteritidis endotoxin (100 micrograms/kg on day 1 and 500 micrograms/kg on day 2). GTP gamma [35S] bound to a single class of sites that were saturable and displaceable in control and endotoxin tolerant macrophage membranes. The maximum specific binding of GTP gamma [35S] was significantly (P < 0.01) decreased in membranes from tolerant rats compared to control (Bmax = 39 +/- 7 pmol/mg protein in control vs. 11 +/- 2 pmol/mg protein in endotoxin tolerant; n = 5). There were no significant differences in the Kd values. To determine whether the reduced GTP gamma S binding was due to decreases in G proteins, macrophage membrane G protein content was determined by western blotting with specific antisera to Gi1,2 alpha, Gi3 alpha, Gs alpha, and the beta subunit of G. Scanning densitometric analysis demonstrated differential decreases in tolerant macrophage membrane G proteins. Gi3 alpha was reduced the most to 48 +/- 8% of controls (n = 3), and this reduction was significant compared to those of other G proteins. Gi1,2 alpha and G beta were reduced to 73 +/- 5% (n = 3) and 65 +/- 4% (n = 3) of control values, respectively. Gs alpha(L) and Gs alpha(H) were reduced to 61 +/- 5% (n = 3) and 68 +/- 3% (n = 3) of control, respectively. These results demonstrate that endotoxin tolerant macrophages exhibit decreased membrane GTP binding capacity and differential reductions in the content of specific G proteins. The cellular mechanisms leading to such alterations in G proteins and their functional significance in the acquisition of endotoxin tolerance merit further investigation.

Characterization of the cloned HEL cell thromboxane A2 receptor: evidence that the affinity state can be altered by G alpha 13 and G alpha q

Thromboxane A2 (TXA2) induces activation of platelets and vascular smooth muscle contraction via cell surface receptors. A platelet type TXA2 receptor from the megakaryocyte-like HEL cell was cloned with a deduced amino acid sequenced identical to that previously reported for the human placental TXA2 receptor. Transient expression of the HEL cell TXA2 receptor cDNA and radioligand binding studies with the agonist 125I-BOP showed a single class of binding sites with an affinity comparable to a low affinity platelet TXA2 receptor. Using a series of 13-azapinane TXA2 analogs, which discriminate between TXA2 receptor subtypes in platelets and vascular smooth muscle, we found that the cloned HEL cell TXA2 receptor is characteristic of a platelet type TXA2 receptor and that its binding characteristics are different from those of vascular smooth muscle cells. The affinity of the HEL cell TXA2 receptor for 125I-BOP was significantly (P < .05) increased upon co-transfection with G alpha 13 alone, or with G alpha q alone and with G alpha 13 and G alpha 12 together (n = 4-6). GTP gamma S significantly (P < .05) decreased the affinity of the receptor for 125I-BOP in COS-7 cell membranes coexpressing HEL-TXR and G alpha 13 to a value comparable to HEL-TXA2 receptor alone. We conclude that 1) the cloned HEL cell TXA2 receptor has pharmacological characteristics of a low affinity platelet type receptor and 2) that the affinity state of this receptor may be influenced by interaction with G alpha 13 and G alpha q.

Activation of thromboxane A2 receptors alters lipopolysaccharide-induced adherence of THP-1 cells

U46619, a thromboxane A2 (TXA2) mimetic, inhibits human monocyte chemotactic responses, suggesting that TXA2, an arachidonic acid metabolite, may alter monocyte adhesion. We tested the hypothesis that TXA2 alters Lipopolysaccharide (LPS)-induced adhesion of THP-1 cells, a human monocytic leukemia cell line. Salmonella enteritidis endotoxin (1 microgram/mL) induced a significant (p < .05; n = 6) increase in the adherence of THP-1 cells (basal, 5.7 +/- 1.8 micrograms/well; LPS, 78.8 +/- 4.9 micrograms/well). Treatment of THP-1 cells with indomethacin or TXA2 receptor antagonists before LPS stimulation significantly (p < .05) enhanced adhesion, suggesting that endogenously produced TXA2 or prostaglandins alter LPS-induced THP-1 cell adhesion. TXA2 mimetics significantly decreased (p < .05; n = 5 and n = 3, respectively) LPS-induced THP-1 cell adhesion. This effect was blocked by three structurally dissimilar TXA2 receptor antagonists. These results support the hypothesis that TXA2 alters LPS-induced adhesion of THP-1 cells.

Androstenedione increases thromboxane A2 receptors in human erythroleukemia cells

Previous studies have demonstrated an increased thromboxane A2 (TXA2) receptor expression in human erythroleukemia (HEL) cells and rat aortic smooth muscle (RASM) cells in response to testosterone treatment. HEL cells have served as a model for megakaryocytes, the progenitor cell for platelets. Platelets have previously been shown to convert androstenedione to testosterone. This study investigated the effects of androstenedione on the TXA2 receptor density in HEL and cultured RASM cells. Both cell lines were incubated with vehicle, 150 nM testosterone or 250, 500 or 750nM androstenedione for 48 hours. Co-incubation with testosterone or androstenedione significantly (p<0.05) increased the maximum number of TXA2 binding sites (Bmax) in HEL cells compared to controls. There was no significant change in Kd values. In a separate series of experiments, HEL cells were incubated with the androgen receptor antagonist hydroxyflutamide (2.5mM). Treatment with androstenedione (500nM) significantly (p<0.05) increased the Bmax value by 35% compared to control and hydroxyflutamide completely antagonized this effect of androstenedione. Incubation with hydroxyflutamide alone had no effect on the Bmax values compared to control. RASM cells also showed an increase in Bmax values by 25% and 23% over control (95+/-6.6, 118+/-7.2 and 117+/-5.1 fmoles/mg protein, control, testosterone and androstenedione, n=3). Both cell lines converted androstenedione to testosterone. The results raise the possibility that the adrenal androgen, androstenedione can regulate the expression of TXA2 receptors either on its own or via conversion to testosterone and through an androgen receptor.

Stimulation of rat mesangial cell thromboxane A2 receptors inhibits particulate but not soluble guanylyl cyclase

Thromboxane A2 (TxA2) participates in the pathogenesis of clinical and experimental glomerular disease. We performed radioligand binding and functional studies of TxA2 receptors in rat mesangial cells. Competitive inhibition of specific binding of the TxA2 analogue [125I]BOP by unlabeled antagonists in intact cells or membranes was observed, with a rank order potency of SQ-29548 (half-maximal inhibitory concentration = 3.4 nM > L-657925 (21 nM) > GR-32191 (200 nM) > L-657926 (1,300 nM). The potency of agonists was I-BOP (0.43 nM) > ONO-11113 (6.7 nM) > U-46619 (80 nM). U-46619 and unlabeled I-BOP inhibited the rate of net guanosine 3',5'-cyclic monophosphate accumulation in cells exposed to atrial natriuretic peptide (ANP) but not the nitric oxide donor nitroprusside. Membranes from cells exposed to I-BOP for 10 min exhibited a 38% decrease in ANP-responsive guanylyl cyclase activity. U-46619 blocked the inhibitory effect of ANP on serum-stimulated [3H]thymidine incorporation but not that of nitroprusside. In summary, we describe a novel effect mediated by a mesangial cell TxA2 receptor, i.e., inhibition of ANP signaling.

Testosterone increases human platelet thromboxane A2 receptor density and aggregation responses

BACKGROUND

The incidence of thrombotic cardiovascular disease is greater in men than in premenopausal women. Testosterone has been implicated as a significant risk factor for cardiovascular disease and for acute myocardial infarctions and strokes in young male athletes who abuse anabolic steroids. Thromboxane A2 (TXA2) is a vasoconstrictor and platelet proaggregatory agent that has been implicated in the pathogenesis of cardiovascular disease. We therefore tested the hypothesis that testosterone regulates the expression of human platelet TXA2 receptors.

METHODS AND RESULTS

In a double-blind, placebo-controlled, randomized, parallel-group study, we determined the effects of testosterone cypionate 200 mg IM given twice, 2 weeks apart, or saline placebo in 16 healthy men. Platelet TXA2 receptor density (Bmax) and dissociation constant (Kd) were measured by use of the TXA2 mimetic 125I-BOP. Platelet aggregation responses to I-BOP and to thrombin and plasma testosterone concentrations were measured before treatment (pretreatment phase), at 2 and 4 weeks (active phase), and again at 8 weeks (recovery phase). Treatment with testosterone was associated with an increase in the Bmax value from 0.95 +/- 0.13 to 2.10 +/- 0.4 pmol/mg protein (n = 9), with a peak effect at 4 weeks (P = .001), returning to baseline by 8 weeks. There was no significant change in Bmax values in the saline-treated group. The Kd values were unchanged. Testosterone treatment was associated with a significant increase in the maximum platelet aggregation response to I-BOP (P < .001) at 4 weeks and returned to baseline at 8 weeks. The EC50 values were not significantly changed. Platelet TXA2 receptor density was positively correlated (r = .56, P < .001, n = 32 measurements) with pretreatment (endogenous) plasma testosterone levels (range, 215 to 883 ng/dL) but not Kd.

CONCLUSIONS

Testosterone regulates the expression of platelet TXA2 receptors in humans. This may contribute to the thrombogenicity of androgenic steroids.

Inhibition of ligand binding to thromboxane A2/prostaglandin H2 receptors by diethylpyrocarbonate. Protection by receptor ligands and reversal by hydroxylamine

The potential of histidines to modulate the binding of agonists and antagonists to human platelet thromboxane A2 (TXA2) receptors was investigated. TXA2 receptors were purified from crude platelet membranes via affinity and wheat germ lectin chromatography. Radioligand binding studies were conducted using the TXA2, mimetic [125I]BOP (I-BOP (I-BOP = [1S-(1 alpha,2 beta(5Z),3 alpha(1E, 3R*),4 alpha)]-7-[3-(3-hydroxy-4-(4'-iodophenoxy)-1-butenyl)7-oxabicyclo- [2.2.1]heptan-2-yl]-5-heptenoic acid) and the TXA2 receptor antagonist [125I]SAP (I-SAP = 7-[(1R,2S,3S,5R)-6,6-dimethyl-3-(4-iodobenzene- sulfonylamino)-bicyclo-[3.1.1]hept-2-yl]-(5Z)-heptenoic acid). The histidine modifying reagent diethyl-pyrocarbonate (DEPC) produced a concentration (30-100 microM) dependent inhibition of binding of both [125I]BOP and [125I]SAP. DEPC treatment significantly (P < 0.05, N = 6) decreased the affinity of the receptor for [125I]SAP (Kd = 2.4 +/- 0.4 and 5.4 +/- 0.4 nM, control and DEPC, respectively) without significantly decreasing the Bmax. The effects of DEPC were reversed by hydroxylamine. The inhibition of [125I]BOP and [125I]SAP binding produced by DEPC was reduced significantly by prior incubation of the purified receptors with the TXA2 receptor agonist U-46619 or the TXA2 receptor antagonist SQ 29548. The results strongly support the notion that one or more histidines reside in a domain that can modulate ligand binding to the TXA2 receptor.

Sex steroid regulation of thromboxane A2 receptors in cultured rat aortic smooth muscle cells

Thromboxane A2 (TXA2) has been implicated as an important mediator of cardiovascular diseases, and male rat aortas are reported to be more sensitive to it than female aortas. The effects of sex steroids to regulate the expression of TXA2 receptors in cultured male rat aortic smooth muscle cells (RASMC) were determined. TXA2 receptor density (Bmax) and affinity (Kd) were determined via radioligand binding studies with [125I]BOP, a TXA2 receptor agonist. Testosterone increased Bmax in a concentration-dependent manner without any significant change in Kd. Cycloheximide, actinomycin D, and the 5 alpha-reductase inhibitor L645,390 significantly (P < 0.01) blocked the effect of testosterone. Dihydrotestosterone, the active metabolite of testosterone, increased Bmax and was more potent than testosterone. To determine if there is a sex-related difference in response to testosterone, its effect in cultured female RASMC was assessed. Testosterone increased Bmax in female RASMC but the increase was significantly (P < 0.001) less than that seen in male RASMC. These results indicate that androgenic steroids regulate the expression of vascular TXA2 receptors.

Increased nitric oxide synthesis during the development of endotoxin tolerance

The role of nitric oxide (NO) synthesis was investigated in endotoxin (LPS) tolerance induced in rats by intraperitoneal injection of a sublethal dose of Salmonella enteritidis LPS (100 micrograms/kg intraperitoneally). Peritoneal macrophages were harvested 6 and 24 h after LPS injection and stimulated in vitro with LPS. LPS significantly stimulated arachidonic acid metabolism, as assessed by 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) levels, and NO production, as assessed by nitrite, in macrophages collected from control rats. In macrophages from tolerant rats LPS-stimulated 6-keto-PGF1 alpha production was significantly reduced, while nitrite production was increased compared to control macrophages (p < .001). In in vivo mortality studies, rats that were pretreated 24 h earlier with sublethal LPS were resistant to the lethal effect of a subsequent dose of LPS (15 mg/kg intravenously) in comparison to control rats (p < .001). NG-Nitro-L-arginine-methyl ester, an inhibitor of NO synthase, decreased mean survival time in control rats and abrogated the resistance to the lethal effect of LPS in tolerant rats. In contrast, molsidomine, a NO donor, improved survival in control rats but did not modify the resistance to the lethal dose of LPS in tolerant rats. The results suggest that sustained NO synthesis may be a beneficial mechanism for the induction of LPS tolerance.

Reorientation of macrophage mediator production in endotoxin tolerance

During endotoxin shock macrophages produce arachidonic acid (AA) metabolites, nitric oxide (NO) and interleukin 6 (IL-6). In contrast, macrophages from endotoxin tolerant rats become hyporesponsive to LPS-induced AA metabolites production. However the role of NO and IL-6 during endotoxin tolerance is not known. Therefore, we evaluated the production of the AA metabolite 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha), IL-6 and NO (by nitrite measurement) by peritoneal macrophages from endotoxin tolerant rats. Since pertussis toxin (PT) sensitive guanine nucleotide binding regulatory (Gi) protein activity is altered during endotoxin tolerance, we also studied the effect of PT on the regulation of the above mediators synthesis. Endotoxin tolerance was induced in rats by intraperitoneal injection of a sublethal dose of Salmonella enteritidis lipopolysaccharide (LPS, 100 micrograms/kg ip). Peritoneal macrophages were harvested 24 hours after LPS injection and stimulated in vitro with LPS (50 micrograms/ml) for determination of NO activity by nitrite, 6-keto-PGF1 alpha and IL-6 production. In macrophages collected from vehicle-pretreated rats (control) LPS stimulates all three mediators. In vivo pretreatment with LPS induced a desensitization of macrophages to LPS-induced 6-keto-PGF1 alpha production compared to control macrophages (p < 0.001). LPS-stimulated IL-6 synthesis was also partially, but not completely, reduced in tolerant macrophages (p < 0.001 versus control).(ABSTRACT TRUNCATED AT 250 WORDS)

Testosterone regulation of platelet and vascular thromboxane A2 receptors

Testosterone has been implicated as a risk factor for cardiovascular diseases and thromboxane A2 (TXA2) plays a role in these diseases. We tested the notion that testosterone regulates the expression of TXA2 receptors in platelets and vascular smooth muscle. Testosterone significantly increased the density of TXA2 receptors in cultured rat aortic smooth muscle and human erythroleukemia cells, a megakaryocyte-like cell. Treatment of rats with testosterone resulted in a significant increase in platelet and aortic TXA2 receptor density and increased responsiveness to TXA2 mimetics. We conclude that testosterone regulates the expression of TXA2 receptors.

Potentiation of PDGF-induced growth responses in coronary artery smooth muscle cells by thromboxane

Mitogenic effects of TXA2 in vascular smooth muscle cells are discussed to be dependent on the age of the donor organism. The present study investigates the contribution of TXA2 on PDGF-induced proliferation of bovine coronary artery smooth muscle cells (BCA-SMC) isolated from adult animals. Radioligand binding studies revealed high affinity TXA2 binding sites (Kd = 1.6 nM) in these cells. TXA2-mimetics alone showed no proliferative effect in BCA-SMC, assessed by [3H]thymidine incorporation. However, PDGF-stimulated proliferation was potentiated two-fold receptor-dependently by TXA2-mimetics. Thus, vasoconstrictory eicosanoids released from activated platelets might aggravate proliferation of vascular smooth muscle cells at sites of vessel injury in the adult organism.

Endotoxin tolerance is associated with decreased prostaglandin H synthases-1 and -2

Lipopolysaccharide (LPS) induces macrophage protein and eicosanoid synthesis. Previous studies have shown that LPS induction of eicosanoid metabolism is transcription dependent and that prostaglandin (PG) H synthase is the committed step in the conversion of arachidonic acid (AA) to thromboxane (Tx) B2. LPS tolerance induced by sublethal in vivo injections of LPS renders rats resistant to LPS in vivo and macrophages refractory to LPS-stimulated eicosanoid synthesis in vitro. This study examined potential activity and content changes in constitutive and mitogen inducible PGH synthase in LPS-stimulated control and tolerant macrophages. TxB2 levels were measured to evaluate basal PGH synthase activity and stimulation by Salmonella enteritidis LPS (50 micrograms/ml), calcium ionophore A-23187, and AA. All tolerant macrophage groups demonstrated decreased TxB2 synthesis. TxB2 synthesis stimulated by AA in tolerant cells was decreased by 70% (P < 0.05) compared with control macrophages. In subsequent studies changes in PGH synthase content were examined in rat peritoneal macrophages from tolerant and control rats incubated with and without LPS. Immunoblot analysis of PGH synthase-1 (constitutive) demonstrated no increase in cells stimulated with LPS compared with basal but was diminished by 62 +/- 9% (n = 4, P < 0.05) in tolerant macrophages compared with control cells. Immunoblot analysis of PGH synthase-2 (mitogen inducible) demonstrated induction in response to LPS that was maximal between 12 and 24 h. PGH synthase-2 was also induced in tolerant macrophages in response to LPS but was less than in control cells. The results demonstrate that endotoxin tolerance is associated with reduced activity and content of PGH synthase-1 and a decreased LPS induction of PGH synthase-2.(ABSTRACT TRUNCATED AT 250 WORDS)

Testosterone increases thromboxane A2 receptor density and responsiveness in rat aortas and platelets

Testosterone has been implicated as a risk factor for cardiovascular diseases. Thromboxane (Tx) A2 is an important pathophysiological mediator for thrombotic vascular diseases. This study investigated the effects of testosterone on platelet and vascular TxA2 receptors. Male rats were treated with either testosterone cypionate for 2 wk, sham operated, castrated, or castrated and treated with testosterone cypionate for 2 wk. Treatment of intact rats with testosterone significantly (P < 0.001) increased the TxA2 receptor density in platelets from 25.4 +/- 3.2 to 42.9 +/- 4.2 fmol/mg protein (P < 0.005, n = 17) and in aortic membranes from 48.7 +/- 1.7 to 86.1 +/- 6.1 fmol/mg protein, n = 9. The threshold concentration of the TxA2 mimetic, [1S-(1 alpha, 2 beta(5Z),3 alpha(1E,3R*)4 alpha)]-7-[3-(3-hydroxy-4- (4'-iodophenoxy)-1-butenyl)-7-oxabicyclo[2.21]heptan-2-yl]-5 -heptenoic acid (I-BOP), to induce platelet aggregation was significantly (P < 0.01) decreased from 0.45 +/- 0.16 nM, n = 7, in the control rats to 0.07 +/- 0.01 nM, n = 13, in the testosterone-treated rats. Testosterone treatment resulted in a significantly (P < 0.05) greater maximum aortic contractile response to the TxA2 mimetic, U-46619, compared with intact rats. Castration resulted in a significant (P < 0.01) decrease in aortic TxA2 receptor density from 51.7 +/- 3.7 to 27.3 +/- 5.3 fmol/mg protein, which was significantly reversed by testosterone treatment (89.2 +/- 7.1 fmol/mg protein; n = 4). Castration resulted in a significantly (P < 0.05) lower maximal aortic contractile response that was reversed by treatment with testosterone. Castration did not significantly change platelet TxA2 receptor density.(ABSTRACT TRUNCATED AT 250 WORDS)

Antiaggregatory activity of 8-epi-prostaglandin F2 alpha and other F-series prostanoids and their binding to thromboxane A2/prostaglandin H2 receptors in human platelets

8-Epi-prostaglandin F2 alpha (8-epi-PGF 2 alpha) is a nonenzymatic, free radical-catalyzed peroxidation product of arachidonic acid that has potent biological activity, including contraction of vasculature and inhibition of aggregation induced by thromboxane (TX) A2 mimetics. In the present study, we demonstrate that 8-epi-PGF2 alpha could inhibit platelet aggregation induced by the TX mimetics U46619 and I-BOP as well as low-dose collagen but not thrombin or the primary wave of aggregation caused by high-dose ADP. The secondary (TX-dependent) wave of aggregation induced by high-dose ADP, however, was not affected. This suppression was dose dependent where 3.6 and 3.3 microM of 8-epi-PGF2 alpha caused 50% inhibition of platelet aggregation induced by U46619 and I-BOP, respectively, whereas 10 microM caused approximately 72% inhibition of collagen-induced aggregation. In contrast, 8-epi-PGF2 alpha significantly potentiated reversible platelet aggregation in response to low-dose ADP. These results indicate that 8-epi-PGF2 alpha has partial agonist activity. 9 alpha,11 beta-PGF2, a structural isomer of 8-epi-PGF2 alpha, inhibited platelet aggregation induced by collagen, high- and low-dose ADP and thrombin, demonstrating marked differences between structural isomers where 9 alpha,11 beta-PGF2 inhibited platelet aggregation induced by TX-dependent as well as TX-independent stimuli. In addition to platelet aggregation, we performed competition-binding assays on washed human platelets using [125I]BOP to further investigate the interaction of 8-epi-PGF2 alpha and 9 alpha,11 beta-PGF2 with TXA2/PGH2 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of human peripheral blood monocyte thromboxane A2 receptors

Thromboxane A2 (TxA2) is the predominant eicosanoid metabolite produced by activated human blood-borne monocytes. This study was designed to characterize TxA2 receptors on human peripheral blood monocytes via identification of radioligand binding characteristics using the stable TxA2 mimetic [125I]-BOP ([1S,(1 alpha,2 beta(5Z),3 alpha(1E,3S*),4 alpha]-7-[3-hydroxy-4-(4'- iodophenoxy-1-butenyl)-7-oxabicyclo-[2.2.1]heptan-2-yl]-5-he ptenoic acid) and via analysis of second messenger signal transduction pathways. Scatchard analysis of the binding of [125I]-BOP in human monocyte membranes revealed a single class of binding sites, Kd = 1.49 +/- 0.14 nM and Bmax = 696 +/- 113 fmol/mg membrane protein (n = 8). [125I]-BOP interacted specifically with a TxA2 receptor, as shown by competition binding studies with a range of TxA2 receptor agonists and antagonists that gave a rank order of potency of (-)-9-chlorobenzyl-6-fluoro-1,2,3,4-tetrahydrocarbazol-1-yl acetic acid > I-BOP = I-SAP > ONO11113 > SQ29548 > U46619 > (+)-9-chlorobenzyl-6-fluoro-1,2,3,4- tetrahydrocarbazol-1-yl acetic acid. I-BOP caused a dose-dependent increase in intracellular free calcium (EC50 = 7.14 +/- 1.1 nM, n = 4), which was attenuated by preincubation with the TxA2 receptor antagonists SQ29548 (1 microM) and (-)-9-chlorobenzyl-6-fluoro-1,2,3,4-tetrahydrocarbazol-1-yl acetic acid (100 nM). This study provides further evidence for a TxA2 receptor in monocytes and supports the potential for future characterization of TxA2 receptor subtypes using molecular techniques.

Decrease in vascular TxA2 receptors in a subgroup of rabbits unresponsive to a TxA2 mimetic

In rabbit pulmonary artery, endothelium-dependent contractions to arachidonic acid and methacholine are mediated by thromboxane (Tx) A2. The TxA2 mimetics U-46619 and (1S-[1 alpha,2 beta(5Z),3 alpha(1E,3R*),4 alpha])-7-[3- [3-hydroxy-4-(4'-iodophenoxy)-1-butenyl]-7-oxabicyclo(2.2.1)hep tan-2-yl]- 5-heptenoic acid (I-BOP), norepinephrine, and endothelin produced endothelium-independent contractions. Arachidonic acid, methacholine, U-46619, and I-BOP failed to produce contractions in a subgroup of rabbits (25%). Nonresponder arteries contracted similarly to norepinephrine and endothelin as responder arteries. The affinity (Kd) and density (Bmax) of TxA2 receptors in crude pulmonary artery membranes were assessed via equilibrium binding studies using 125I-BOP. There was no difference in Kd between the two groups (0.49 +/- 0.17 vs. 0.32 +/- 0.14 nM, responder vs. nonresponder). There was a significant decrease in Bmax (123 +/- 21 vs. 28 +/- 11 fmol/mg protein, responder vs. nonresponder; P < 0.01) of receptors in the nonresponders. Nonresponder aortas also did not contract to U-46619 and exhibited a decrease in TxA2 receptors, indicating that the difference is not specific for the pulmonary artery. Nonresponder platelets aggregated to U-46619, suggesting that the platelet receptor is not altered. TxA2-receptor expression may be regulated in vivo. Nonresponder rabbits may provide a useful model for studying these receptors in vascular disease.

Endotoxin tolerance: effects on lethality and macrophage thromboxane (B2) and interleukin 6 production

The effects of endotoxin pretreatment on induction of in vivo tolerance to endotoxin lethality, and on in vitro stimulated peritoneal macrophage mediators, thromboxane (TX)B2 and interleukin 6 (IL-6) were investigated. Rats were given i.p. injections of S. enteritidis endotoxin on days 1 (100 micrograms/kg) and 2 (500 micrograms/kg), respectively. After 5 days, or after 2-8 weeks of initial pretreatment, either endotoxin-induced mortality was assessed, or peritoneal cells were harvested for the in vitro studies. Endotoxin tolerant rats were resistant (p < .05) to endotoxin lethality for 2 weeks after initial induction of tolerance. In vitro studies with peritoneal macrophages demonstrated that endotoxin or monophosphoryl lipid A stimulated (p < .05) TXB2 production. However, peritoneal cells harvested from endotoxin tolerant rats exhibited suppressed (*p < .05) TXB2 production to both stimuli which persisted for at least 8 weeks. Endotoxin stimulated (p < .05) in vitro levels of IL-6 in control cells, but in contrast to the suppressed TXB2 production in tolerance, also stimulated (p < .05) in vitro IL-6 in the endotoxin tolerant group. Paradoxically, lipid A did not induce IL-6 production in either group. These composite observations suggest that during endotoxin tolerance neither in vitro peritoneal macrophage TXB2 nor IL-6 synthesis temporally correlate with in vivo resistance to lethality.

Agonist-induced phosphorylation of human platelet TXA2/PGH2 receptors

Thromboxane A2 (TXA2) and its precursor prostaglandin H2 (PGH2) stimulate platelet activation through interaction with TXA2/PGH2 receptors. We and others have shown that these receptors undergo homologous desensitization upon prolonged exposure to thromboxane A2 mimetics. Phosphorylation of receptors has previously been reported to be an important mechanism for receptor desensitization. In the present study we examined the possibility that homologous desensitization of human platelet TXA2/PGH2 receptors may involve phosphorylation. The ATP pool of human platelets was metabolically prelabeled with 32Pi and the labeled platelets were subsequently exposed for 1 and 10 min to the stable TXA2/PGH2 mimetic, U46619 (1 microM). TXA2/PGH2 receptors were purified approx. 2000-fold by affinity and wheatgerm lectin chromatography and subjected to SDS-PAGE followed by autoradiography. A phosphorylated plasma membrane glycoprotein (M(r) = 50-57 kDa) was detected with characteristics similar to the TXA2/PGH2 receptor. This glycoprotein was found to be phosphorylated in the unstimulated state but phosphorylation was increased by exposure to U46619. Phosphorylation occurred rapidly and was inhibited when platelets were preincubated with the TXA2/PGH2 receptor antagonist, SQ29548 (5 microM), before being stimulated with U46619. These results suggest that human platelet TXA2/PGH2 receptors are phosphoproteins and that the level of phosphorylation is increased during homologous desensitization.

Testosterone treatment enhances thromboxane A2 mimetic induced coronary artery vasoconstriction in guinea pigs

Thromboxane A2 (TXA2) has been implicated as an important mediator of cardiovascular diseases. There have been several clinical reports of acute myocardial infarctions occurring in young male athletes abusing anabolic steroids. The effects of treatment of male Guinea pigs with testosterone on the responses to U46619, a TXA2 receptor agonist, in the isolated perfused heart were determined. The maximum pressor responses of the isolated perfused Guinea pig heart to U46619 were significantly (P < 0.05) greater in the Guinea pigs treated with testosterone compared to the controls. These results indicate that testosterone can enhance coronary artery vascular reactivity to TXA2.

Thromboxane A2 receptor blockade improves renal function and histopathology in the post-obstructive kidney

To elucidate the role of the vasoconstrictor thromboxane A2(TXA2) in post-obstructive nephropathy, we examined the effect of the TXA2 receptor antagonist GR32191(GR) on renal function and histopathology in the post-obstructed kidney (POK) in rats. Rats pre-treated with 3 or 6 mg/kg i.p. of GR prior to ureteral obstruction and maintained on b.i.d. doses of GR were compared to vehicle-treated and sham-operated controls. Renal hemodynamic, clearance and excretory function were assessed in each kidney following relief of 24 hours of unilateral ureteral obstruction. The histology of each kidney was evaluated. Mean clearances of inulin for the POK were significantly greater in the treated rats (0.42 +/- 0.06 ml/min at 6 mg/kg) than in controls (0.13 +/- 0.04 ml/min) and a dose-response effect was observed (P < 0.05). Paraaminohippurate clearance was increased by > 150% and renal vascular resistance was reduced by 50% in GR treated animals compared with controls (P < 0.05). Histopathologic findings in the untreated POK were typical of early obstruction. In the GR treated groups these changes were much less severe. These data support an important role for TXA2 in the pathogenesis of post-obstructive nephropathy.

The Gordon Wilson Lecture. Regulation of thromboxane A2 receptors by testosterone: implications for steroid abuse and cardiovascular disease

Thromboxane A2 (TXA2), a platelet aggregator and vasoconstrictor, has been implicated as a potential pathophysiologic mediator of a wide variety of cardiovascular diseases. It is well established that men are at greater risk for cardiovascular disease compared to premenopausal females. Abuse of androgenic/anabolic steroids has been associated with thrombotic cardiovascular diseases in young male athletes. These observations along with several others have led to the hypothesis that testosterone may regulate the expression of TXA2 receptors. Rat aortic smooth muscle cells (RASMC) and human erythroleukemia cells (HEL), a megakaryocyte-like cell, were incubated with testosterone. TXA2 receptor affinity (Kd) and density (Bmax) were determined via equilibrium binding experiments using the radiolabeled TXA2 mimetic [125I]-BOP. Testosterone significantly increased the Bmax without any significant change in Kd. Hydroxyflutamide (1 microM), an androgen receptor antagonist, completely blocked the effect of testosterone. Dihydrotestosterone, the active metabolite of testosterone also increased Bmax in a concentration-dependent manner and was more potent than testosterone. These observations along with several others are consistent with the notion that androgenic steroids may regulate the expression of functional TXA2 receptors in HEL and RASMC. These results raise the possibility that the increase in TXA2 receptor density induced by testosterone may contribute to its thrombotic potential in cardiovascular diseases.