Thromboxane A2/prostaglandin H2 receptors in streptozotocin-induced diabetes: effects of insulin therapy in the rat

Downregulation of Thromboxane A2 Receptor Occurs Mainly via Nuclear Factor-KappaB Signaling Pathway in Rat Renal Artery

Thromboxane A₂ (TXA₂) acts on TXA₂ receptors (TP) to regulate renal artery blood flow and subsequently contributes to the pathogenesis of renal ischemia. The present study was designed to examine if nuclear factor-kappaB (NF-κB) signaling pathway is involved in the downregulation of TP receptors in rat renal artery. Rat renal artery segments were organ cultured for 6 or 24 h. Downregulation of TP receptors was monitored using myograph (contractile function), real-time PCR (receptor mRNA), and immunohistochemistry (receptor protein). Specific inhibitors (MG-132 and BMS345541) for NF-κB signaling pathway were used to dissect the underlying molecular mechanisms involved. Compared to fresh (noncultured) segments, organ culture of the renal artery segments for 24 h induced a significant rightward shift of U46619 (TP receptor agonist) contractile response curves (pEC₅₀: 6.89 ± 0.06 versus 6.48 ± 0.04, P < 0.001). This decreased contractile response to U46619 was paralleled with decreased TP receptor mRNA and protein expressions in the renal artery smooth muscle cells. Specific inhibitors (MG-132 and BMS345541) for NF-κB signaling pathway significantly abolished the decreased TP protein expression and receptor-mediated contractions. In conclusion, downregulation of TP receptors in the renal artery smooth muscle cells occurs mainly via the NF-κB signaling pathway.

Effects of endothelin receptor blockade on hypervasoreactivity in streptozotocin-diabetic rats: vessel-specific involvement of thromboxane A2

Increased vasoconstrictor response to norepinephrine (NE) and endothelin (ET)-1 in arteries from diabetic animals is ameliorated by chronic endothelin receptor blockade with bosentan and was absent in endothelium-denuded arteries, suggesting the involvement of ET-1 and an endothelium-derived contracting factor such as thromboxane A2 (TxA2). To examine this possibility, we determined the effects of acute blockade of ET receptors or inhibition of TxA2 synthesis on the vascular function of superior mesenteric arteries (SMA) and renal arteries (RA) isolated from nondiabetic and 11-week streptozotocin (STZ) diabetic rats chronically treated with either bosentan or vehicle. Both in vitro incubation with bosentan and a selective ETA receptor blocker, BQ123, eradicated the increase in NE contractile responses in diabetic SMA. Additionally, in vitro incubation with the thromboxane synthase inhibitor, dazmegrel, abrogated the exaggerated NE and ET-1 contractile responses in diabetic SMA. Conversely, in RA, no significant acute effect of bosentan, BQ123, nor dazmegrel on vascular responses to NE was observed. Dazmegrel incubation attenuated the maximum contractile responses to ET-1 in diabetic RA; however, these responses in diabetic RA remained significantly greater than those of other groups. Diabetic RA but not SMA exhibited an enhanced contractile response to the TxA2 analogue U46619, which was corrected by chronic bosentan treatment. Immunohistochemical analyses in diabetic SMA revealed an increase in ETA receptor level that was normalized by chronic bosentan treatment. These data indicate that an interaction between ET-1 and TxA2 may be involved in mediating the exaggerated vasoconstrictor responses in diabetic arteries. Furthermore, the underlying mechanisms appear to be vessel specific.

Endothelial nitric oxide synthase-dependent tyrosine nitration of prostacyclin synthase in diabetes in vivo

There is evidence that reactive nitrogen species are implicated in diabetic vascular complications, but their sources and targets remain largely unidentified. In the present study, we aimed to study the roles of endothelial nitric oxide synthase (eNOS) in diabetes. Exposure of isolated bovine coronary arteries to high glucose (30 mmol/l d-glucose) but not to osmotic control mannitol (30 mmol/l) switched angiotensin II-stimulated prostacyclin (PGI(2))-dependent relaxation into a persistent vasoconstriction that was sensitive to either indomethacin, a cyclooxygenase inhibitor, or SQ29548, a selective thromboxane receptor antagonist. In parallel, high glucose, but not mannitol, significantly increased superoxide and 3-nitrotyrosine in PGI(2) synthase (PGIS). Concurrent administration of polyethylene-glycolated superoxide dismutase (SOD), l-nitroarginine methyl ester, or sepiapterin not only reversed the effects of high glucose on both angiotensin II-induced relaxation and PGI(2) release but also abolished high-glucose-enhanced PGIS nitration, as well as its association with eNOS. Furthermore, diabetes significantly suppressed PGIS activity in parallel with increased superoxide and PGIS nitration in the aortas of diabetic C57BL6 mice but had less effect in diabetic mice either lacking eNOS or overexpressing human SOD (hSOD(+/+)), suggesting an eNOS-dependent PGIS nitration in vivo. We conclude that diabetes increases PGIS nitration in vivo, likely via dysfunctional eNOS.

Effects of diabetes on the vascular response to nitric oxide and constrictor prostanoids: gender and regional differences

To analyze the effects of diabetes mellitus on the vascular responsiveness to nitric oxide and thromboxane receptor stimulation, 2 mm long segments of basilar, coronary, renal and tail arteries from male and female, control (normoglycemic) and streptozotocin-induced diabetic rats, were prepared for isometric tension recording. In the segments at basal resting tension, the thromboxane analog U46619 (10(-9)-10(-5) M) produced concentration-dependent contraction, which was similar in arteries from male and female rats, and was reduced by diabetes in coronary arteries from male and in tail arteries from female rats. In the vascular segments precontracted with endothelin-1 (10(-9) M), acetylcholine (10(-9)-3 x 10(-5) M) produced concentration-dependent relaxation which was similar in all arteries from normoglycemic male and female rats, and was increased by diabetes in tail arteries from female, but not in those from male rats. In precontracted segments the nitric oxide donor sodium nitroprusside (10(-10)-10(-5) M) also produced concentration-dependent relaxation, which was higher in basilar arteries from normoglycemic females compared with males, and was increased by diabetes in tail arteries from female but not from male rats. These results suggest that diabetes may increase the relaxation to nitric oxide in tail arteries, and may reduce the contraction to thromboxane receptor activation in coronary and tail arteries in a gender-dependent way. These changes in vascular reactivity may be adaptative to the vascular alterations produced by diabetes.

A thromboxane A(2) system in the Atlantic stingray, Dasyatis sabina

Thromboxane B(2)(TXB(2)) is the stable metabolite of thromboxane A(2)(TXA(2)) and thromboxane B(2)-like immunoreactivity (iTXB(2)) has been identified in the plasma of the Atlantic stingray, Dasyatis sabina (0.57+/-0.03 ng/ml). Plasma levels of iTXB(2) increase if the blood is allowed to clot (3.0+/-0.27 ng/ml). When clotting occurs in the presence of indomethacin, this increase is partially inhibited (1.5+/-0.17 ng/ml), indicating the presence of a cyclooxygenase activity. Radioligand binding analysis using the TXA(2) analog [125I]BOP in isolated kidney membranes revealed a receptor of K(d)=2.88+/-0.51 nM and B(max)=25.6+/-5.9 fmol/mg protein. [125I]BOP binding was displaced by the TXA(2) receptor (TP receptor) agonists U46619 (IC(50)=106.4+/-15.7 nM) and U44069 (IC(50)=88.7+/-13.0 nM), and the antagonist SQ29548 (IC(50)=51.0+/-12.9 nM). Binding was also displaced stereoselectively by the antagonists (-)L657925 (IC(50)=18.9+/-3.8 nM) and (+)L657926 (IC(50)=2025+/-280 nM). Tissue bath studies revealed that U46619, a stable TXA(2) mimetic, elicited concentration-dependent contractions in the ventral aorta which were inhibited in a concentration-dependent manner by the TP receptor antagonist SQ29548. Using a human TP receptor riboprobe, Northern blotting of mRNA isolated from the stingray kidney identified transcripts of 2.8 and 6kb. The 2.8kb transcript is similar to a 2.8kb transcript found in human cells or tissues, but the 6kb transcript may be unique. These data indicate the presence of a TXB(2)-like substance in the blood, a TP receptor in the kidney, TXA(2) biological activity in the ventral aorta, and expression of a TP receptor-like gene.

Role of endothelin-1 and thromboxane A2 in renal vasoconstriction induced by angiotensin II in diabetes and hypertension

BACKGROUND

Interactions among angiotensin II (Ang II), endothelin-1 (ET-1) and thromboxane A2 (TXA2) may play an important role in the regulation of renal function. The present study investigated the participation of TXA2 and ET-1 in the increase in renal vascular resistances (RVR) induced by Ang II, as well as the consequences of diabetes, hypertension, and the combination of both on this response.

METHODS

Isolated kidneys from male normoglycemic or streptozotocin-induced diabetic Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) were used. The increase in perfusion pressure (PP) produced by Ang II was studied in the absence or presence of the TXA2 receptor antagonist, ifetroban, or the ETA/ETB receptor antagonist, PD145065.

RESULTS

Systolic arterial pressure (SAP) was higher in SHR than in WKY, but diabetic rats (D) from each strain showed lower SAP values than their respective non-diabetic rats. Basal renal PP was higher in WKY and SHR than in WKY-D and SHR-D. Increases in renal PP produced by Ang II were comparable in the kidneys from all groups. Either ifetroban or PD145065 reduced the maximal Ang II response in all animals. The maximal inhibitory effect of ifetroban was higher (P<0.05) in WKY than in the other groups. However, the maximal inhibitory effect of PD145065 was lower in SHR than in the other groups.

CONCLUSION

This study supports a role for ET-1 and TXA2 as mediators of the increase in renal vascular resistance produced by Ang II. These results indicate that the participation of ET-1 in the renal vasoconstriction produced by Ang II was reduced under hypertensive conditions, and that of TXA2 was reduced by both diabetes and hypertension.

G protein-coupled prostanoid receptors and the kidney

Renal cyclooxygenase 1 and 2 activity produces five primary prostanoids: prostaglandin E2, prostaglandin F2alpha, prostaglandin I2, thromboxane A2, and prostaglandin D2. These lipid mediators interact with a family of distinct G protein-coupled prostanoid receptors designated EP, FP, IP, TP, and DP, respectively, which exert important regulatory effects on renal function. The intrarenal distribution of these prostanoid receptors has been mapped, and the consequences of their activation have been partially characterized. FP, TP, and EP1 receptors preferentially couple to an increase in cell calcium. EP2, EP4, DP, and IP receptors stimulate cyclic AMP, whereas the EP3 receptor preferentially couples to Gi, inhibiting cyclic AMP generation. EP1 and EP3 mRNA expression predominates in the collecting duct and thick limb, respectively, where their stimulation reduces NaCl and water absorption, promoting natriuresis and diuresis. The FP receptor is highly expressed in the distal convoluted tubule, where it may have a distinct effect on renal salt transport. Although only low levels of EP2 receptor mRNA are detected in the kidney and its precise intrarenal localization is uncertain, mice with targeted disruption of the EP2 receptor exhibit salt-sensitive hypertension, suggesting that this receptor may also play an important role in salt excretion. In contrast, EP4 receptor mRNA is predominantly expressed in the glomerulus, where it may contribute to the regulation of glomerular hemodynamics and renin release. The IP receptor mRNA is highly expressed near the glomerulus, in the afferent arteriole, where it may also dilate renal arterioles and stimulate renin release. Conversely, TP receptors in the glomerulus may counteract the effects of these dilator prostanoids and increase glomerular resistance. At present there is little evidence for DP receptor expression in the kidney. These receptors act in a concerted fashion as physiological buffers, protecting the kidney from excessive functional changes during periods of physiological stress. Nonsteroidal anti-inflammatory drug (NSAID)-mediated cyclooxygenase inhibition results in the loss of these combined effects, which contributes to their renal effects. Selective prostanoid receptor antagonists may provide new therapeutic approaches for specific disease states.

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.

Expression and tissue distribution of the mRNAs encoding the human thromboxane A2 receptor (TP) alpha and beta isoforms

The human thromboxane A2 receptor (TP), a G protein-coupled receptor, exists as two isoforms, TPalpha and TPbeta, which arise by alternative mRNA splicing and differ exclusively in their carboxyl terminal cytoplasmic regions. In this study, a reverse transcriptase-polymerase chain reaction (RT-PCR)-based strategy was developed to examine the expression of the TPs in tissues of physiologic relevance to TXA2. Although most of the 17 different cell/tissue types examined expressed both TP isoforms, the liver hepatoblastoma HepG2 cell line was found to exclusively express TPalpha mRNA. In most cell types, TPalpha mRNA predominated over TPbeta mRNA. Moreover, although the levels of TPalpha mRNA expression were similar in most of the cell/tissue types examined, extensive differences in the levels of TPbeta mRNA were observed. Consequently, the relative expression of TPalpha: TPbeta mRNA varied considerably due to extensive differences in TPbeta mRNA expression. Most strikingly, primary HUVECs were found to express: (i) low levels of TPbeta and (ii) approximately 6-fold greater levels of TPalpha than TPbeta. These data were confirmed in the spontaneously transformed HUVEC derived ECV304 cell line. Expression of TP mRNAs in the various tissue/cells correlated with protein expression, as assessed by radioligand binding using the selective TP antagonist [3H]SQ29,548.

Increased susceptibility of gastric mucosa to ulcerogenic stimulation in diabetic rats--role of capsaicin-sensitive sensory neurons

1. We examined the gastric mucosal blood flow (GMBF) and ulcerogenic responses following barrier disruption induced by sodium taurocholate (TC) in diabetic rats and investigated the role of capsaicin-sensitive sensory neurons in these responses. 2. Animals were injected streptozotocin (STZ: 70 mg kg(-1), i.p.) and used after 5, 10 and 15 weeks of diabetes with blood glucose levels of > 350 mg dl(-1). The stomach was mounted on an ex-vivo chamber under urethane anaesthesia and exposed to 20 mM TC plus 50 mM HCl for 30 min in the presence of omeprazole. Gastric transmucosal potential difference (PD), GMBF, and luminal acid loss (H+ back-diffusion) were measured before and after exposure to 20 mM TC, and the mucosa was examined for lesions 90 min after TC treatment. 3. Mucosal application of TC caused PD reduction in all groups; the degree of PD reduction was similar between normal and diabetic rats, although basal PD values were lower in diabetic rats. In normal rats, TC treatment caused luminal acid loss, followed by an increase of GMBF, resulting in minimal damage in the mucosa. 4. The increased GMBF responses associated with H+ back-diffusion were mitigated in STZ-treated rats, depending on the duration of diabetes, and severe haemorrhagic lesions occurred in the stomach after 10 weeks of diabetes. 5. Intragastric application of capsaicin increased GMBF in normal rats, but such responses were mitigated in STZ diabetic rats. The amount of CGRP released in the isolated stomach in response to capsaicin was significantly lower in diabetic rats when compared to controls. 6. The deleterious influences on GMBF and mucosal ulcerogenic responses in STZ-diabetic rats were partially but significantly antagonized by daily insulin (4 units rat(-1)) treatment. 7. These results suggest that the gastric mucosa of diabetic rats is more vulnerable to acid injury following barrier disruption, and this change is insulin-sensitive and may be partly accounted for by the impairment of GMBF response associated with acid back-diffusion and mediated by capsaicin-sensitive sensory neurons.

Tyrosine phosphorylation of phosphatidylinositol 3-kinase and of the thromboxane A2 (TXA2) receptor by the TXA2 mimetic I-BOP in A7r5 cells

Thromboxane A2 (TXA2) interacts with its G-protein coupled receptor, the TP receptor, to produce contraction and proliferation of vascular smooth muscle cells. We have shown previously that proliferation of primary cultures of vascular smooth muscle cells initiated by [1S-(1alpha, 2beta(5Z), 3alpha(1E, 3R), 4alpha]-7-[3-(3-hydroxy-4-(4'-iodophenoxy)-1-butenyl)-7-oxab icyclo-[2.2.1]heptan-2yl]-5'-heptenoic acid (I-BOP), a stable TXA2 mimetic, is mediated by activation of mitogen-activated protein (MAP) kinase. In the present study, we examined further the intracellular mediators involved in TXA2 activation of vascular smooth muscle cells. Transient transfection of the cDNA for the TP receptor into A7r5 vascular smooth muscle cells resulted in expression of TP receptors with a receptor density, Bmax, of 0.7 +/- 0.2 pmol/mg protein and a receptor affinity, Kd, of 0.6 +/- 0.1 nM (N = 7). Mock transfected cells lacked significant receptor expression. In TP receptor transfected cells, I-BOP increased the activation of MAP kinase 2-fold, stimulated tyrosine phosphorylation of cellular proteins of relative molecular mass (Mr) of 140, 85, 60, 56, and 45 kDa, and increased the message for c-jun, a nuclear transcription factor involved in mitogenesis, 2.6-fold. Immunoblot analysis indicated that the 85-kDa protein represented phosphoinositide 3-kinase (PI3-K), while the 60 kDa protein was the TP receptor. The activity of PI3-K was increased 3.5-fold by the addition of I-BOP (0.1 microM). In summary, the present study demonstrated that stimulation of the TP receptor results in tyrosine phosphorylation of the receptor and of PI3-K.