Testing the "redirection hypothesis" of prostaglandin metabolism in the kidney

Thromboxane A2 modulates cisplatin-induced apoptosis through a Siva1-dependent mechanism

Thromboxane A(2) (TXA(2)) is an important lipid mediator whose function in apoptosis is the subject of conflicting reports. Here, a yeast two-hybrid screen for proteins that interact with the C-terminus of the TXA(2) receptor (TP) identified Siva1 as a new TP-interacting protein. Contradictory evidence suggests pro- and anti-apoptotic roles for Siva1. We show that a cisplatin treatment induces TXA(2) synthesis in HeLa cells. We demonstrate that endogenous TP stimulation promotes cisplatin-induced apoptosis of HeLa cells and that such modulation requires the expression of Siva1, as evidenced by inhibiting its endogenous expression using siRNAs. We reveal that, upon stimulation of TP, degradation of Siva1 is impeded, resulting in an accumulation of the protein, which translocates from the nucleus to the cytosol. Translocation of Siva1 correlates with its reduced interaction with Mdm2 (an inhibitor of p53 signalling), as well as with its increased interaction with TRAF2 and XIAP (known to enhance pro-apoptotic signalling). Our data provide a model that reconciles the pro- and anti-apoptotic roles that were reported for Siva1 and identify a new mechanism for promoting apoptosis by G protein-coupled receptors. Our findings may have implications in the use of cyclo-oxygenase inhibitors during cisplatin chemotherapy and might provide a target to reduce cisplatin toxicity on non-cancerous tissues.

Effects of ridogrel, a thromboxane synthase inhibitor and receptor antagonist, on blood pressure in the spontaneously hypertensive rat

Ridogrel is a dual acting thromboxane synthase inhibitor/TP receptor antagonist. We examined the effects of single and multiple doses on systolic blood pressure in stroke-prone spontaneously hypertensive rats. Single doses of ridogrel (5 to 125 mg/kg) did not affect systolic blood pressure or furosemide-stimulated excretion rates of thromboxane B2 or 6-keto-prostaglandin F1alpha, although ex vivo serum thromboxane B2 was dose-dependently reduced up to 95%. In contrast, repeated dosing (7 days) with ridogrel (3 to 25 mg/kg/day), had an antihypertensive effect in 12-week-old stroke-prone spontaneously hypertensive rats. At 25 mg/kg/day, ridogrel reduced systolic blood pressure from 200+/-6.1 to 173+/-6.7 mmHg (n=12, P<0.01). Ridogrel dose-dependently reduced serum thromboxane B2 and increased plasma renin activity. Unlike single doses, repeated dosing reduced urinary thromboxane B2 excretion (from 103+/-7 ng/day to 49+/-10 ng/day, P<0.01) while preserving 6-keto-prostaglandin F1alpha excretion. Ketoprofen, a cyclo-oxygenase inhibitor, (10 mg/kg/day for 7 days), depressed urine 6-keto-prostaglandin F1alpha in addition to attenuating serum and urine thromboxane B2. Ketoprofen prevented the antihypertensive effects of ridogrel. Ridogrel did not lower systolic blood pressure in Sprague-Dawley rats. We conclude that the antihypertensive effect of ridogrel involves preserving renal prostaglandin synthesis during thromboxane attenuation.

Thromboxane is required for full expression of angiotensin hypertension in rats

Recent studies suggest that thromboxane (TX) mediates a significant component of angiotensin II (ANG II)-induced hypertension. However, there is little information to support the hypothesis that this relationship is important during chronic, physiological increases in ANG II, particularly while controlling for variation in endogenous ANG II levels induced by TX inhibition. This study tested that hypothesis in 27 chronically instrumented rats. After baseline measurements, suppression of endogenous TX was induced and maintained throughout the study in 13 rats by i.v. infusion of the TX synthesis inhibitor (TSI) U63557A: the other 14 rats received vehicle. Baseline mean arterial pressure (MAP) was not different between groups and was unchanged by TSI or vehicle. Continuous inhibition of ANG II production was then initiated in both groups of rats by i.v. infusion of the angiotensin-converting enzyme inhibitor (ACEI) benazepril. ACEI reduced blood pressure similarly in vehicle and TSI rats, from 105 +/- 2 to 91 +/- 2 mm Hg and 103 +/- 1 to 89 +/- 1 mm Hg, respectively. ANG II was then infused at 5 ng.kg-1.min-1 i.v. for 7 days in six rats from each group to restore ANG II activity to baseline levels. This dose increased MAP to 103 +/- 2 and 101 +/- 1 mm Hg in vehicle and TSI rats, respectively, values not different from pre-ACEI levels. Seven TSI rats and eight vehicle rats received a higher dose of ANG II (20 ng.kg-1.min-1 i.v.). After 7 days, MAP was higher in vehicle than in TSI rats (143 +/- 5 versus 120 +/- 4 mm Hg). These results suggest that endogenous TX is an important determinant of MAP in ANG II hypertension but may have a diminished role in blood pressure regulation when ANG II is at normal and subnormal levels.

Thromboxane and prostacyclin differentially regulate murine extracellular matrix gene expression

Alterations in the arachidonic acid metabolites thromboxane and prostacyclin are known to contribute to hemodynamic changes observed in certain models of acute and chronic renal failure. We have previously shown that thromboxane may have an important role in mediating glomerulosclerosis by stimulating the expression of certain extracellular matrix proteins. In the present study, we compared the effects of thromboxane and prostacyclin on the expression of genes encoding basement membrane proteins using a murine teratocarcinoma cell line, that when differentiated to an endodermal phenotype synthesizes abundant extracellular matrix. Incubation of these cells with stable analogs of thromboxane and prostacyclin for four hours resulted in changes in basement membrane gene expression. Thromboxane increased steady-state mRNA levels for all three laminin chains, type IV collagen, and fibronectin, but decreased the level of mRNA for heparan sulfate proteoglycan. In contrast, incubation with carbo-prostacyclin, a stable analog of prostacyclin, decreased the steady-state mRNA level for the laminin A and B1 chains, type IV collagen and fibronectin, and increased the mRNA level for heparan sulfate proteoglycan and laminin B2. Carbo-prostacyclin did not affect cellular proliferation or thymidine incorporation. These results indicate that eicosanoids directly modulate matrix gene expression independently of hemodynamic influence, and independently of effects mediated by platelets, or mitogenesis. Furthermore, these findings suggest that the alterations in renal eicosanoid metabolism may directly participate in the pathogenesis of glomerulosclerosis and thus provide a rationale for therapy directed toward the specific inhibition of thromboxane in the treatment of progressive glomerular sclerosis.