Thromboxane binding and signal transduction in rat glomerular mesangial cells

Diverse associations between oxidative stress and thromboxane A2 in hypertensive glomerular injury

We examined the potential contributions of oxidative stress and thromboxane A2 (TXA2) to the development of regional heterogeneity in hypertensive glomerular injury using stroke-prone spontaneously hypertensive rats (SHRSP), an animal model of human essential hypertension. We also examined the effect of antioxidant treatment on the regional expression of thromboxane synthase (TXAS) mRNA using a microdissection method. Increases in the glomerular expression of TXAS mRNA were observed in the SHRSP at 15 weeks of age compared with those in the age-matched normotensive control Wistar-Kyoto (WKY) rats: 2.4-fold and 3.1-fold in the superficial and juxtamedullary glomeruli, respectively (P < 0.05). The heme oxygenase-1 mRNA expression was markedly increased (greater than eightfold, P < 0.05) in both the superficial and juxtamedullary glomeruli in the SHRSP compared with the expression in the WKY rats. In contrast to our expectations, the treatment of SHRSP with tempol (a superoxide dismutase mimetic) significantly (P < 0.05) increased the TXAS mRNA expression in the superficial glomeruli and did not improve the histological injury or albuminuria, which were both aggravated. Moreover, ozagrel (a TXAS inhibitor) had a suppressive effect on the TXAS mRNA expression and significantly (P < 0.05) improved the histological injury. These results indicated that although TXA2 and oxidative stress are linked to each other, TXA2 rather than oxidative stress may be a better therapeutic target to improve hypertensive glomerular injury.

EP1- and FP-mediated cross-desensitization of the alpha (alpha) and beta (beta) isoforms of the human thromboxane A2 receptor

1. Heterologous desensitization or intermolecular cross-talk plays a critical role in regulating intracellular signalling by diverse members of the G-protein-coupled receptor superfamily. We have previously established that the alpha and beta isoforms of the human thromboxane A(2) receptor (TP) undergo differential desensitization of signalling in response to 17 phenyl trinor prostaglandin (PG)E(2), an agonist of the EP(1) subtype of the PGE(2) receptor (EP) family. 2. Herein, we investigated the molecular basis of TPalpha and TPbeta desensitization in human embryonic kidney (HEK) 293 cells and in renal mesangial cells in response to 17 phenyl trinor PGE(2) and in response to the PGF(2alpha) receptor (FP) agonist PGF(2alpha), and sought to identify the target site(s) of those desensitizations. 3. Our results demonstrated that TPalpha and TPbeta receptors are subject to desensitization in response to both EP(1) and FP receptor activation and that these effects are mediated by direct protein kinase (PK)C phosphorylation of the individual TP isoforms within their unique carboxyl-terminal (C)-tail domains. 4. Moreover, deletion/site-directed mutagenesis and metabolic labelling studies identified Thr(337), within TPalpha, and Thr(399), within TPbeta, as the specific target residues for PKC phosphorylation and EP(1)- and FP-mediated desensitization of TPalpha and TPbeta signalling, respectively. 5. Hence, in conclusion, while the TPalpha and TPbeta diverge within their C-tail domains, they have evolved to share a similar mechanism of PKC-induced phosphorylation and desensitization in response to EP(1) and FP receptor activation, though it occurs at sites unique to the individual TP isoforms.

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.

5-HT(2A) receptors stimulate mitogen-activated protein kinase via H(2)O(2) generation in rat renal mesangial cells

Serotonin (5-HT) stimulates mitogenesis in rat renal mesangial cells through a G protein-coupled 5-HT(2A) receptor. We tested the hypothesis that oxidants might be involved in the signal transduction pathway linking the receptor to extracellular signal-regulated protein kinase (ERK). 5-HT rapidly increased the activity and phosphorylation of ERK. These effects were blocked by the 5-HT(2A) receptor antagonist ketanserin. The peak effect was noted at 5-10 min, and half-maximal stimulation was achieved at 10-30 nM 5-HT. Chemical inhibitor and activator studies supported the involvement of phospholipase C, protein kinase C (PKC), and reactive oxygen species (ROS, i.e., H(2)O(2) and superoxide) generated by an NAD(P)H oxidase-like enzyme in the ERK activation cascade. Mapping studies supported a location for the NAD(P)H oxidase enzyme and the ROS downstream from PKC. Our studies are most consistent with an ERK activation pathway as follows: 5-HT(2A) receptor --> G(q) protein --> phospholipase C --> diacylglycerol --> classical PKC --> NAD(P)H oxidase --> superoxide --> superoxide dismutase --> H(2)O(2) --> mitogen-activated extracellular signal-regulated kinase --> ERK. These studies demonstrate a role for the 5-HT(2A) receptor in rapid, potent, and efficacious activation of ERK in rat renal mesangial cells. They support a role for oxidants in conveying the stimulatory signal from 5-HT, because 1) chemical antioxidants attenuate the 5-HT signal, 2) oxidants and 5-HT selectively activate ERK to a similar degree, 3) 5-HT produces superoxide and H(2)O(2) in these cells, and 4) a specific enzyme [NAD(P)H oxidase] has been implicated as the source of the ROS, which react selectively downstream of classical PKC.

Thromboxane receptors in human kidney tissues

Thromboxane (TX) A2 effects in the kidneys include contraction of glomerular mesangial cells and intrarenal vascular tissue. A kidney cDNA encoding a TX receptor expressed in rat renal glomeruli and rat renal arterial smooth muscle cells has been reported. However, TXA2 receptors in human kidneys have not been documented. The purpose of this study was to identify and characterize TXA2 receptors in glomeruli and intrarenal arteries isolated from human kidneys. Normal kidneys, not used for transplant because of technical reasons, were kept at -70 degrees C and used for research purposes. The glomeruli and intrarenal arteries were isolated from renal cortical tissue by a mechanical sieving technique. The equilibrium dissociation constant and receptor number were determined by nonlinear analysis of binding inhibition data. The data were generated in radioreceptor assays using [125I]-BOP, a stable analog of TXA2. The dissociation constants (mean +/- SEM) for binding of I-BOP to human glomeruli and intrarenal arterial membranes were 6.6 +/- 1.1 nM (n = 7) and 20 +/- 6 nM (n = 7), respectively (p < 0.05). The receptor number was 311 +/- 91 fmol/mg protein (n = 7) in glomeruli and 74 +/- 16 fmol/mg protein (n = 7) in intrarenal arterial membranes (p < 0.04). The order of specificity of TXA2 analogs for [125I]-BOP binding sites was similar in glomeruli and in arterial membranes and was I-BOP > or = U46619 > or = pinane TXA2 > or = carbocyclic TXA2 > or = PGH2. These findings provide direct evidence for the presence of specific, high-affinity [125I]-BOP binding sites in human renal glomeruli and extraglomerular vascular tissue. These data also indicate that the human binding sites have higher affinity for the TXA2 agonist I-BOP than for PGH2.

Coagulation defects and altered hemodynamic responses in mice lacking receptors for thromboxane A2

Thromboxane A2 (TXA2) is a labile metabolite of arachidonic acid that has potent biological effects. Its actions are mediated by G protein-coupled thromboxane-prostanoid (TP) receptors. TP receptors have been implicated in the pathogenesis of cardiovascular diseases. To investigate the physiological functions of TP receptors, we generated TP receptor-deficient mice by gene targeting. Tp-/- animals reproduce and survive in expected numbers, and their major organ systems are normal. Thromboxane agonist binding cannot be detected in tissues from Tp-/- mice. Bleeding times are prolonged in Tp-/- mice and their platelets do not aggregate after exposure to TXA2 agonists. Aggregation responses after collagen stimulation are also delayed, although ADP-stimulated aggregation is normal. Infusion of the TP receptor agonist U-46619 causes transient increases in blood pressure followed by cardiovascular collapse in wild-type mice, but U-46619 caused no hemodynamic effect in Tp-/- mice. Tp-/- mice are also resistant to arachidonic acid-induced shock, although arachidonic acid signifi-cantly reduced blood pressure in Tp-/- mice. In summary, Tp-/- mice have a mild bleeding disorder and altered vascular responses to TXA2 and arachidonic acid. Our studies suggest that most of the recognized functions of TXA2 are mediated by the single known Tp gene locus.

Effect of receptor number on desensitization of the mouse thromboxane receptor

Desensitization of G-protein coupled receptors limits the physiologic effects of an agonist. Short-term desensitization mechanisms are critically dependent on receptor phosphorylation by protein kinases. The effectiveness of these regulatory mechanisms might be limited by substrate (receptor) availability. To investigate the role of receptor number in the desensitization of G-protein coupled receptors, we transfected a mouse mesangial cell line with a genomic clone encoding the mouse thromboxane A2 (TxA2) receptor and obtained cell lines that expressed low (approximately 250-500 fmol/mg protein) or high (2500-4000 fmol/mg protein) levels of TxA2 receptors. Activation of TxA2 receptors stimulated phosphoinositide (PI) hydrolysis and increased intracellular calcium ([Ca2+]i) levels. Prior exposure to the TxA2 agonist (15S)-hydroxy-11alpha,9alpha-(epoxymethano)prosta-5Z,+ ++13E-dienoic acid (U46619) reduced subsequent (15S)-hydroxy- 11alpha,9alpha-(epoxymethano)prosta-5Z,13E-dieno ic acid-induced increases in inositol trisphosphates and intracellular calcium levels by approximately 50% in clones expressing low numbers of TxA2 receptors, but had little effect on TxA2 receptor responsiveness in clones expressing high receptor numbers. Failure of TxA2 receptors to desensitize caused sustained increases in intracellular calcium levels and phosphoinositide hydrolysis. Thus, homologous desensitization of TxA2 receptors is attenuated in cells expressing high levels of receptors for TxA2. These data suggest that receptor number plays a key role in the short-term regulation of G-protein coupled receptors.

Rat kidney thromboxane receptor: molecular cloning, signal transduction, and intrarenal expression localization

Thromboxane (TX) plays important roles in control of renal hemodynamics and water and electrolyte metabolism, and is involved in the pathophysiology of many renal diseases. The aim of the present study is to isolate a rat kidney cDNA encoding functional TX receptor, and to reveal its intrarenal expression localization. A clone (rTXR2) was isolated from a rat kidney cDNA library by a homology screening approach. rTXR2 was shown to encode the amino acid sequence containing seven transmembrane spanning domains representing rat (r) TX receptor. The membrane from COS-7 cells transiently transfected with rTXR2 cDNA was shown to be specifically bound by a thromboxane receptor antagonist, SQ29548. Either in Xenopus oocyte expression or in transfected COS-7 cells, rTX receptor was shown to be linked with Ca2+ messenger system. TX receptor-mediated increase in cytosolic Ca2+ was also observed in cultured glomerular mesangial cells. In situ hybridization showed that rTX receptor mRNA was detected in renal glomeruli, smooth muscle cells in renal arterioles, and transitional cell epithelium of renal pelvis. Reverse transcription linked to PCR applied to microdissected nephron segments indicated the presence of rTX receptor mRNA exclusively in the glomerulus. In conclusion, we have cloned a functional rat kidney TX receptor, which is expressed specifically in renal glomerulus, arterial smooth muscle cells, and transitional cell epithelium of renal pelvis. The present study will provide important insights into the etiology and pathophysiology of renal diseases with relation to TX metabolism.