Localization of authentic thromboxane A2/prostaglandin H2 receptor in the rat kidney

Increased thromboxane/prostaglandin receptors contribute to high glucose-induced podocyte injury and mitochondrial fission through ROCK1-Drp1 signaling

Excessive mitochondrial fission in podocytes serves as a central hub for the pathogenesis of diabetic nephropathy (DN), and the thromboxane/prostaglandin receptor (TP receptor) plays a potential role in DN. However, regulation of the TP receptor during mitochondrial dynamics disorder in podocytes remains unknown. Here, we firstly reported novel mechanistic details of TP receptor effects on mitochondrial dynamics in podocytes under diabetic conditions. Expression of the TP receptor was significantly upregulated in podocytes under diabetic conditions both in vivo and in vitro. S18886 attenuated podocyte mitochondrial fission, glomerular injury and renal dysfunction in diabetic mice. Furthermore, inhibition of the TP receptor by both genetic and pharmacological methods dramatically reduced mitochondrial fission and attenuated podocyte injury induced by high glucose through regulating dynamin-related protein 1 (Drp1) phosphorylation and its subsequent translocation to mitochondria. In contrast, TP receptor overexpression and application of TP receptor agonist U46619 in these podocytes showed the opposite effect on mitochondrial fission and podocyte injury. Furthermore, treatment with Y27632, an inhibitor of Rho-associated kinase1 (ROCK1), significantly blunted more fragmented mitochondria and reduced podocyte injuries in podocytes with TP receptor overexpression or after U46619 treatment. Finally, pharmacological inhibition of Drp1 alleviated excessive mitochondrial fragmentation and podocyte damage in TP receptor overexpressing podocytes. Our data suggests that increased expression of the TP receptor can occur in a human cultured podocyte cell line and in podocytes derived from streptozotocin (STZ)-induced diabetic mice, which contributes to mitochondrial excessive fission and podocyte injury via ROCK1-Drp1 signaling.

Glomerular Biomechanical Stress and Lipid Mediators during Cellular Changes Leading to Chronic Kidney Disease

Hyperfiltration is an important underlying cause of glomerular dysfunction associated with several systemic and intrinsic glomerular conditions leading to chronic kidney disease (CKD). These include obesity, diabetes, hypertension, focal segmental glomerulosclerosis (FSGS), congenital abnormalities and reduced renal mass (low nephron number). Hyperfiltration-associated biomechanical forces directly impact the cell membrane, generating tensile and fluid flow shear stresses in multiple segments of the nephron. Ongoing research suggests these biomechanical forces as the initial mediators of hyperfiltration-induced deterioration of podocyte structure and function leading to their detachment and irreplaceable loss from the glomerular filtration barrier. Membrane lipid-derived polyunsaturated fatty acids (PUFA) and their metabolites are potent transducers of biomechanical stress from the cell surface to intracellular compartments. Omega-6 and ω-3 long-chain PUFA from membrane phospholipids generate many versatile and autacoid oxylipins that modulate pro-inflammatory as well as anti-inflammatory autocrine and paracrine signaling. We advance the idea that lipid signaling molecules, related enzymes, metabolites and receptors are not just mediators of cellular stress but also potential targets for developing novel interventions. With the growing emphasis on lifestyle changes for wellness, dietary fatty acids are potential adjunct-therapeutics to minimize/treat hyperfiltration-induced progressive glomerular damage and CKD.

Urinary thromboxane B2 and thromboxane receptors in bladder cancer: opportunity for detection and monitoring

We have previously found increased expression of thromboxane synthase (TXAS) and thromboxane receptor (TP) beta isoform in the tissues of patients with bladder cancer. Studies in cell lines and mice have indicated a potential significant role of the thromboxane signaling pathway in the pathogenesis of human bladder cancer. This study was designed to determine if the changes observed in the tissues of patients with bladder cancer were mirrored by changes in the urine of these patients. We found increased levels of thromboxane B(2) (TXB(2)) the major metabolite of TXAS and increased levels of the TPβ receptor. These results raised the possibility that patients with bladder cancer may be followed for progression or remission of their disease by quantitation of these substances in their urine.

Allergic inflammation induces a persistent mechanistic switch in thromboxane-mediated airway constriction in the mouse

Actions of thromboxane (TXA(2)) to alter airway resistance were first identified over 25 years ago. However, the mechanism underlying this physiological response has remained largely undefined. Here we address this question using a novel panel of mice in which expression of the thromboxane receptor (TP) has been genetically manipulated. We show that the response of the airways to TXA(2) is complex: it depends on expression of other G protein-coupled receptors but also on the physiological context of the signal. In the healthy airway, TXA(2)-mediated airway constriction depends on expression of TP receptors by smooth muscle cells. In contrast, in the inflamed lung, the direct actions of TXA(2) on smooth muscle cell TP receptors no longer contribute to bronchoconstriction. Instead, in allergic lung disease, TXA(2)-mediated airway constriction depends on neuronal TP receptors. Furthermore, this mechanistic switch persists long after resolution of pulmonary inflammation. Our findings demonstrate the powerful ability of lung inflammation to modify pathways leading to airway constriction, resulting in persistent changes in mechanisms of airway reactivity to key bronchoconstrictors. Such alterations are likely to shape the pathogenesis of asthmatic lung disease.

Thromboxane A2: physiology/pathophysiology, cellular signal transduction and pharmacology

Thromboxane A(2) (TXA(2)), an unstable arachidonic acid metabolite, elicits diverse physiological/pathophysiological actions, including platelet aggregation and smooth muscle contraction. TXA(2) has been shown to be involved in allergies, modulation of acquired immunity, atherogenesis, neovascularization, and metastasis of cancer cells. The TXA(2) receptor (TP) communicates mainly with G(q) and G(13), resulting in phospholipase C activation and RhoGEF activation, respectively. In addition, TP couples with G(11), G(12), G(13), G(14), G(15), G(16), G(i), G(s) and G(h). TP is widely distributed in the body, and is expressed at high levels in thymus and spleen. The second extracellular loop of TP is an important ligand-binding site, and Asp(193) is a key amino acid. There are two alternatively spliced isoforms of TP, TPalpha and TPbeta, which differ only in their C-terminals. TPalpha and TPbeta communicate with different G proteins, and undergo hetero-dimerization, resulting in changes in intracellular traffic and receptor protein conformations. TP cross-talks with receptor tyrosine kinases, such as EGF receptor, to induce cell proliferation and differentiation. TP is glycosylated in the N-terminal region for recruitment to plasma membranes. Furthermore, TP conformation is changed by coupling to G proteins, showing several states of agonist binding. Finally, several drugs modify TP-mediated events; these include cyclooxygenase inhibitors, TXA(2) synthase inhibitors and TP antagonists. Some flavonoids of natural origin also have TP receptor antagonistic activity. Recent advances in TP research have clarified TXA(2)-mediated events in detail, and further study will supply more beneficial information about TXA(2) pathophysiology.

Prostaglandins in the kidney: developments since Y2K

There are five major PGs (prostaglandins/prostanoids) produced from arachidonic acid via the COX (cyclo-oxygenase) pathway: PGE(2), PGI(2) (prostacyclin), PGD(2), PGF(2alpha) and TXA(2) (thromboxane A(2)). They exert many biological effects through specific G-protein-coupled membrane receptors, namely EP (PGE(2) receptor), IP (PGI(2) receptor), DP (PGD(2) receptor), FP (PGF(2alpha) receptor) and TP (TXA(2) receptor) respectively. PGs are implicated in physiological and pathological processes in all major organ systems, including cardiovascular function, gastrointestinal responses, reproductive processes, renal effects etc. This review highlights recent insights into the role of each prostanoid in regulating various aspects of renal function, including haemodynamics, renin secretion, growth responses, tubular transport processes and cell fate. A thorough review of the literature since Y2K (year 2000) is provided, with a general overview of PGs and their synthesis enzymes, and then specific considerations of each PG/prostanoid receptor system in the kidney.

Expression and hormonal modulation of the thromboxane A2 receptor gene in mammalian testicular arteries

OBJECTIVE

Thromboxane A2 (TXA2) is a biologically potent arachidonate metabolite which causes vasoconstriction and platelet aggregation. This study aimed to evaluate the presence of TXA2 receptors in mammalian testicular arteries and the influence of exogenous gonadotropin administration on their expression.

DESIGN

Experimental study.

SETTING

Experimental animals in an academic research environment.

ANIMAL(S)

Seven rams aged 4-6 years, weighing 60-90 kg.

INTERVENTION(S)

Both testicular arteries of seven rams were extracted before (from one testis) and after (from the opposite testis) IV administration of 5,000 IU human chorionic gonadotropin (hCG).

MAIN OUTCOME MEASURE(S)

The expression of the TXA2 receptor gene was investigated by the real-time polymerase chain reaction. Data were analyzed by means of Student t test. Results were expressed as mean +/- standard deviation. A P value of <.01 was considered statistically significant.

RESULT(S)

Polymerase chain reaction revealed the presence of TXA2 messenger RNA receptor in all the basal samples, but the expression of TXA2 receptor gene was undetectable in all the arteries obtained 24 h after the administration of hCG.

CONCLUSION(S)

These preliminary results demonstrated for the first time the presence of TXA2 receptors in mammalian testicular arteries. Results also showed that their expression was down-regulated after hCG administration.

Effects of isoprostane on tubuloglomerular feedback: roles of TP receptors, NOS, and salt intake

A thromboxane prostanoid receptor (TP-R) agonist U-46,619 enhances tubuloglomerular feedback (TGF). Glomerular expression of TP-R and enhancement of TGF by U-46,619 increase with salt intake. We investigated the hypothesis that 8-isoprostaglandin F(2alpha) (8-Iso) activates TGF via TP-R. The maximal TGF response in rats was assessed from the fall in proximal stop flow pressure (PSF; an index of glomerular capillary pressure) during loop of Henle (LH) microperfusion of artificial tubular fluid (ATF) at 40 nl/min. Microperfusion of 8-Iso (10(-4) M) into the efferent arteriole (EA) enhanced TGF responses by 20 +/- 3% (P < 0.01). TGF response to 8-Iso was independent of dietary salt [DeltaTGF%, low salt (LS): 21 +/- 5%; normal salt (NS): 17 +/- 4%; high salt (HS): 29 +/- 8%, not significant (ns)], unlike the salt-dependent effect of U-46,619 (DeltaTGF%, LS: 41 +/- 5%; NS: 52 +/- 4%; HS: 112 +/- 21%). Ifetroban, the TP-R antagonist, abolished TGF responses to 8-Iso and U-46,619 at all levels of salt intake. During luminal perfusion of N-monomethyl-l-arginine (l-NMA), the effect of 8-Iso on TGF was enhanced in NS and HS but not in LS (LS: 22 +/- 6 vs. LS + l-NMA: 28 +/- 6%, ns; NS: 18 +/- 4 vs. NS + l-NMA: 40 +/- 4, P < 0.01; HS: 27 +/- 3 vs. HS + l-NMA: 65 +/- 6, P < 0.01). However, U-46,619 did not further increase TGF after l-NMA in all salt groups (LS: 43 +/- 7 vs. LS + l-NMA: 51 +/- 6, ns; NS: 52 +/- 7 vs. NS + l-NMA: 48 +/- 8, ns; HS: 114 +/- 21 vs. HS + l-NMA: 74 +/- 22, ns). In conclusion, activation of TP receptors by U-46,619 and 8-Iso-PGF(2alpha) enhances TGF. In addition, the effect of U-46,619 was salt dependent, whereas the effect of 8-Iso-PGF(2alpha) was salt independent. However, stimulation of NO by 8-isoprostanes masks its salt-sensitive effect on TGF.

Thromboxane synthase and TP receptor mRNA in rat kidney and brain: effects of salt intake and ANG II

A TP receptor (TP-R) mimetic causes salt-sensitive hypertension and renal afferent arteriolar vasoconstriction. TP-Rs mediate effects of ANG II on renal vascular resistance and drinking. Therefore, we investigated the hypothesis that thromboxane A(2) synthase (TxA(2)-S) and/or TP-R expression is regulated by salt and/or ANG II. Rats (n = 6) received high-salt (HS) or low-salt (LS) diets. Additional HS-diet rats received ANG II while other HS- and LS-diet rats received the AT(1) receptor (AT(1)-R) antagonist losartan. Excretion of thromboxane B(2) by conscious rats was increased with the HS diet compared with the LS diet (126 +/- 10 vs. 48 +/- 5 pmol/24 h, respectively; P < 0.01). The mRNA abundance for TP-Rs (relative to beta-actin) in the kidney cortex was enhanced 30% by the HS diet (P < 0.001) and was reduced 50% by the addition of ANG II (P < 0.001). However, during losartan administration, the effects of salt were reversed; mRNA more than doubled during the LS diet (P < 0.001). Similarly, the mRNA abundance for TP-Rs in the brain stem was reduced by 50% with the addition of ANG II (P < 0.001) and during losartan administration was almost doubled by the LS diet (P < 0.001). The mRNA abundance for TxA(2)-S in the kidney cortex also was increased many times with the HS diet (P < 0.001). In contrast, the mRNA for TxA(2)-S in the brain was unaffected by salt. ANG II did not affect TxA(2)-S at either site. During losartan administration, TxA(2)-S increased modestly in the brain stem with the LS diet. mRNA abundance for TP-Rs in the kidney cortex and brain stem is suppressed by ANG II acting on AT(1)-Rs. In the absence of AT(1)-Rs, expression of TP-Rs at both sites is enhanced by LS intake. In contrast, ANG II does not affect the mRNA abundance for TxA(2)-S. Expression of TxA(2)-S is enhanced by HS intake in the kidney cortex but by LS intake in the brain stem only during losartan administration. Thus TP-Rs are strongly dependent on ANG II acting on AT(1)-Rs, whereas TxA(2)-S is regulated differentially in the kidney cortex and brain stem by salt intake.

Molecular biology of blood pressure regulatory genes

Blood pressure is determined by vascular resistance and circulating volume. Activation of vascular angiotensin II or thromboxane receptor is mostly involved in the former, and function of renal prostagalandin EP3 receptor or thiazide-sensitive sodium-chloride co-transporter is also in the latter. We have cloned rat genes for these blood pressure regulatory factors, and studied their gene expression. Here we review the molecular biology of those genes, based on our observations.

Pharmacological and molecular biological (RT-PCR) characterization of functional TP prostanoid receptors in immortalized human non-pigmented ciliary epithelial cells

Immortalized human non-pigmented ciliary epithelial (NPE) cells (ODM-2) were shown to express the mRNA for the prostanoid TPalpha but not the TPbeta receptor using reverse transcription-polymerase chain reaction (RT-PCR). These TPalpha receptors were coupled to phospholipase C (PLC) and, thus, promoted phosphoinositide (PI) turnover. TP receptor agonists yielded the following potencies (EC50S) in the PI turnover assays: I-BOP = 8.2 +/- 1.1 nM; carbocyclic TA2 = 87.5 +/- 25.3 nM; U-44069 = 1.16 +/- 0.32 microM; U-46619 = 1.2 +/- 0.2 microM (n = 4-17). Agonists selective for other prostanoid receptor subtypes (e.g., fluprostenol and sulprostone) were inactive. The agonist effects of U-44619 and I-BOP were potently blocked, in an apparent non-competitive manner (ki = 53.9 +/- 12 nM; pA2s = 7.6-7.8; pKbs = 7.38), by the TP receptor-selective antagonist, SQ29,548, but were unaffected by other prostanoid receptor antagonists (e.g., AH6809, AL-8810). The PLC inhibitor (U73122) inhibited U-46619-induced PI turnover (IC50 = 4.3 +/- 0.6 microM). The functional potencies of the compounds stimulating or inhibiting the TP receptor-mediated PI turnover in the NPE cells correlated well with the TP receptor binding affinities of these compounds at human platelet TP receptors (r = 0.98). These studies have shown the presence of the mRNA for and the expression of functional TPalpha receptors coupled to PLC in human NPE cells. The TPalpha receptors on NPE cells may be responsible for inhibiting aqueous humor production and may help explain the intraocular pressure-lowering effects of certain TP agonists.

Vasoconstrictor responses in thromboxane receptor knockout mice: tubuloglomerular feedback and ureteral obstruction

The role of thromboxane (TP) in the vasoconstriction induced by tubuloglomerular feedback or 18-h ureteral obstruction was studied in wild type mice (TP +/+), and in heterozygous (TP +/-) and homozygous TP receptor knockout mice (TPR -/-). TGF function was assessed from the response of stop flow pressure (PSF) to a maximum increase in loop of Henle flow rate (0-30 nL min-1). PSF fell by 6.4 +/- 0.4 mmHg in wild-type mice, by 6.1 +/- 0.6 mmHg in TP +/-, and by 7.9 +/- 0.7 mmHg in TP -/- mice. In the presence of the TP receptor agonist U46,619 (10-5 M) the PSF reduction increased to 10. 4 +/- 0.8 mmHg in TP +/+, and to 10.6 +/- 2.8 mmHg in TP +/-, but was unchanged at 7.7 +/- 0.7 mmHg in TP -/-. Mean arterial blood pressures were comparable between groups (103 +/- 3 mmHg in TP +/+, 113 +/- 4.6 in TP +/- and 113 +/- 2.4 mmHg in TP -/- mice). Intratubular pressure following unilateral ureteral obstruction was significantly higher in TP -/- than in TP +/+ mice both in the early phase (0-3 h) and late phase (18 h) of obstruction. These results indicate that chronic TP receptor deficiency does not significantly alter maximum TGF responses in mice, and that it is accompanied by exaggerated vasodilatation during short-term unilateral ureteral obstruction and attenuated vasoconstriction during longer lasting obstruction. We conclude that thromboxane is primarily a regulator of renal vascular tone under pathophysiological conditions.

Changes in glomerular thromboxane A2 receptor expression and ligand binding following immune injury

BACKGROUND

Thromboxane (Tx) A2 is a potent vasoconstrictor eicosanoid that attains high levels within nephritic glomeruli and mediates a drop in glomerular filtration rate (GFR). In the course of nephritis, however, GFR recovers despite high intraglomerular TxA2 levels. We hypothesized that this recovery indicates a reduced responsiveness of the glomerular vasculature to TxA2, and explored whether changes in TxA2 receptor protein expression and receptor-ligand binding are underlying mechanisms.

METHODS

Glomerulonephritis was induced in male Sprague-Dawley rats using an antibody raised in rabbits against rat particulate glomerular basement membrane (GBM). Changes in Tx receptor levels were assessed in protein lysates of glomeruli on days 3 and 7 after a single intravenous injection of the anti-GBM antibody. Ligand-binding studies were performed at the same time points using isolated glomeruli and the TxA2 receptor ligand [3H]-SQ-29,548. GFR was measured as the clearance of endogenous creatinine.

RESULTS

There was a marked increase in Tx receptor protein in the lysates of nephritic glomeruli on days 3 and 7. In contrast, binding sites (Bmax) of [3H]-SQ-29,548 decreased, indicating that the excess receptor became either inaccessible to its ligand (sequestered) or desensitized. Daily administration of the Tx synthase inhibitor Furegrelate starting prior to injection of anti-GBM antibody prevented the decrease in [3H]-SQ-29,548 binding. Furegrelate treatment starting in an established stage of nephritis had no effect. In these animals, GFR was lower than nephritic controls not treated with Furegrelate.

CONCLUSIONS

These observations indicate that in the course of glomerulonephritis, there is a marked increase in glomerular Tx receptor expression. The enhanced intraglomerular TxA2 synthesis causes either a sequestration or desensitization of its receptor. As a result, access of unbound TxA2 to efferent arterioles may become facilitated, and constriction of these arterioles may preserve GFR.

The identification and characterization of oligodendrocyte thromboxane A2 receptors

The presence of functional thromboxane A2 receptors in neonatal rat oligodendrocytes and human oligodendroglioma cells was investigated using immunocytochemistry, ligand affinity chromatography, radioligand binding analysis, immunoblot analysis, and calcium mobilization studies. Immunocytochemical studies revealed the presence of receptor protein on both oligodendrocytes and human oligodendroglioma cells. Ligand affinity chromatography allowed for the purification of a protein with an electrophoretic mobility (55 kDa) indistinguishable from human platelet thromboxane A2 receptors. This affinity purified protein was immunoreactive against a polyclonal anti-thromboxane A2 receptor antibody. Intact human oligodendroglioma cells specifically bound [3H]SQ29,548 with a KD of 4 nM and were found to have approximately 3500 binding sites per cell. Human oligodendroglioma cells also demonstrated calcium mobilization in response to receptor activation with U46619. These results demonstrate the presence of a functional thromboxane A2 receptor in oligodendrocytes and are consistent with previous observations indicating a high density of thromboxane A2 receptors in myelinated brain and spinal cord fiber tracts.

Thromboxane A2 interferes with a disposal process of aggregated protein in glomeruli

Immune complexes in glomeruli are involved in development of diverse glomerulonephritis. The disposal process of glomerular immune complexes has been unclarified. The present studies were undertaken to determine if thromboxane A2 (TXA2) is associated with the disposal of macromolecules in the glomeruli using mice injected with aggregated bovine serum albumin (a-BSA). A-BSA promptly accumulated in the glomeruli, the level reaching a plateau at 6 hr after the injection of a-BSA, and then decreased by 48 hr. The production of glomerular TXA2, prostaglandin E2 (PGE2) and prostaglandin I2 concomitantly increased with the decrease of a-BSA in the glomeruli. TXA2 synthase inhibitors and TXA2 receptor antagonists accelerated clearance of glomerular a-BSA without enhancing renal tissue blood flow. They did not affect a-BSA level in the plasma. In contrast, aminophylline, dopamine and mannitol significantly increased renal tissue blood flow, but did not decrease glomerular a-BSA. TXA2 synthase inhibitors decreased TXA2 production in the glomeruli. TXA2 synthase inhibitors and TXA2 receptor antagonists did not influence the generation of PGE2. The TXA2 analogue U-46619 significantly increased the accumulation of a-BSA in the glomeruli. We propose that TXA2 interferes with the disposal process of aggregated protein in the glomeruli. We also postulate that interception of glomerular activity of TXA2 may be an effective intervention for managing immune complex-mediated glomerulonephritis and glomerulosclerosis.

Salt loading enhances rat renal TxA2/PGH2 receptor expression and TGF response to U-46,619

The tubuloglomerular feedback (TGF) response is potentiated by thromboxane A2 (TxA2) and/or prostaglandin endoperoxide (PGH2) acting on specific receptors. Infusion of the TxA2/PGH2 mimetic, U-46,619, into conscious rats leads to hypertension that is potentiated by a high-salt intake. Therefore, we tested the hypothesis that a high-salt intake enhances the expression of transcripts for TxA2/PGH2 receptors in the kidney and glomeruli and enhances the response of TGF to TxA2/PGH2 receptor stimulation. Groups of rats were accommodated to a low-salt (LS), normal salt (NS), or high-salt (HS) diet for 8-10 days. TxA2/PGH2 receptor mRNA was detected by reverse transcription-polymerase chain reaction in kidney cortex, isolated glomeruli, and abdominal aorta. TxA2/PGH2 mRNA abundance was significantly (P < 0.001) increased during intake of high-salt compared with low-salt diets in the kidney cortex (1.34 +/- 0.10 vs. 0.84 +/- 0.04 arbitrary units) and isolated outer cortical glomeruli (0.68 +/- 0.04 vs. 0.32 +/- 0.03 arbitrary units), but there was no effect of salt on TxA2/PGH2 receptor mRNA expression in the aorta. Maximal TGF responses were assessed from the increase in proximal stop flow pressure (an index of glomerular capillary pressure) during increases in loop of Henle perfusion with artificial tubular fluid from 0 to 40 nl/min. Compared with vehicle, the enhancement of maximal TGF with U-46,619 (10(-6) M) added to the perfusate was greater in rats adapted to high-salt than normal salt (HS: +9.6 +/- 1.1 vs. NS: +5.1 +/- 0.4 mmHg; P < 0.001) or low-salt (LS: +3.8 +/- 1.3 mmHg; P < 0.001) intakes. Responses to U-46,619 at each level of salt intake were blocked by > 70% by the TxA2/PGH2 receptor antagonist ifetroban. In contrast, enhancement of TGF by peritubular capillary perfusion of arginine vasopressin (AVP; 10(-7) M) was similar in high-salt and low-salt rats (HS: +1.5 +/- 0.6 vs. LS: +1.6 +/- 0.5 mmHg; not significant). We conclude that salt loading increases selectively the abundance of TxA2/PGH2 receptor transcripts in the kidney cortex and glomerulus, relative to the aorta, and enhances selectively TGF responses to TxA2/PGH2 receptor activation but not to AVP.