Altered TP receptor function in isolated, perfused kidneys of nondiabetic and diabetic ApoE-deficient mice

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.

Obeticholic acid ameliorates hepatorenal syndrome in ascitic cirrhotic rats by down-regulating the renal 8-iso-PGF2α-activated COX-TXA2 pathway

BACKGROUNDS/AIMS

The present study explores the potential of chronic treatment with the Foresaid X receptor (FXR) agonist obeticholic acid (OCA), which inhibits oxidative stress-related pathogenesis, in ascitic cirrhotic rats with hepatorenal syndrome (HRS) developed 6 weeks after bile duct ligation (BDL).

METHODS

Systemic, splanchnic, and renal hemodynamics and pathogenic cascades were measured in ascitic BDL and sham rats receiving 2-weeks of either vehicle or OCA treatments (sham-OCA and BDL-OCA groups), and NRK-52E cells, rat kidney tubular epithelial cells.

RESULTS

Chronic OCA treatment significantly normalized portal hypertension, glomerular filtration rate, urine output, renal blood flow; decreased ascites, renal vascular resistance, serum creatinine, and the release of renal tubular damage markers, including urinary neutrophil gelatinase-associated lipocalin (uNGAL) and kidney injury moleculae-1 (uKim-1) in BDL-OCA rats. In the BDL group, inhibition of the renal oxidative stress (8-iso-PGF2α)-activated cyclooxygenase-thromboxane A2 [COX-TXA2] pathway, apoptosis, and tubular injury accompanied by a decrease in hyper-responsiveness to the vasoconstrictor 8-iso-PGF2α in perfused kidneys. In vitro experiments revealed that 8-iso-PGF2α induced oxidative stress, release of reactive oxygen species, and cell apoptosis, which were reversed by concomitant incubation with the FXR agonist.

CONCLUSIONS

Through the inhibition of renal 8-iso-PGF2α production and the down-regulation of the COX-TXA2 pathway, our study suggests that chronic OCA treatment can ameliorate the HRS in ascitic cirrhotic rats. Thus, OCA is an agent with antioxidative stress, antivasoconstrictive, antiapoptotic properties which benefit ascitic, cirrhotic rats with systemic, hepatic, and renal abnormalities.

Differential properties of E prostanoid receptor-3 and thromboxane prostanoid receptor in activation by prostacyclin to evoke vasoconstrictor response in the mouse renal vasculature

Vasomotor reactions of prostacyclin (prostaglandin I₂ ; PGI₂ ) can be collectively modulated by thromboxane prostanoid receptor (TP), E-prostanoid receptor-3 (EP3), and the vasodilator I prostanoid receptor (IP). This study aimed to determine the direct effect of PGI₂ on renal arteries and/or the whole renal vasculature and how each of these receptors is involved. Experiments were performed on vessels or perfused kidneys of wild-type mice and/or mice with deficiency in TP (TP^-/- ) and/or EP3. Here we show that PGI₂ did not evoke relaxation, but instead resulted in contraction of main renal arteries (from ~0.001-0.01 µM) or reduction of flow in perfused kidneys (from ~1 µM); either of them was reversed into a dilator response in TP^-/- /EP3^-/- counterparts. Also, we found that in renal arteries although it has a lesser effect than TP^-/- on the maximal contraction to PGI₂ (10 µM), EP3^-/- but not TP^-/- resulted in relaxation to the prostanoid at 0.01-1 µM. Meanwhile, TP^-/- only significantly reduced the contractile activity evoked by PGI₂ at ≥0.1 µM. These results demonstrate that PGI₂ may evoke an overall vasoconstrictor response in the mouse renal vasculature, reflecting activities of TP and EP3 outweighing that of the vasodilator IP. Also, our results suggest that EP3, on which PGI₂ can have a potency similar to that on IP, plays a major role in the vasoconstrictor effect of the prostanoid of low concentrations (≤1 µM), while TP, on which PGI₂ has a lower potency but higher efficacy, accounts for a larger part of its maximal contractile activity.

Effects of thromboxane prostanoid receptor deficiency on diabetic nephropathy induced by high fat diet and streptozotocin in mice

Diabetic nephropathy (DN), one of the main causes of end-stage renal disease, still remains as a challenge of clinical management. This study aimed to determine whether deficiency of the thromboxane (TX) prostanoid receptor (TP), which mediates the contractile activities of all prostanoids, alleviates the development of DN and if so, to examine the underlying mechanism(s). Diabetes was induced by high fat diet and streptozotocin injection in wild-type (WT) mice and those with TP deficiency (TP^-/-). Here we show that WT and TP^-/- mice developed diabetes with a similar blood glucose level; however, signs of renal functional impairments and pathologies occurred to a lesser extent in TP^-/- than in WT mice. Also, the extent of an increase in the expression level of transforming growth factor-β1 (TGF-β1), a common pathological mediator of DN, in diabetic renal cortexes of TP^-/- mice was lower than that of WT counterparts. Moreover, we noted that expression levels of cyclooxygenase (COX)-2 and calcium-dependent phospholipase A₂ (cPLA₂) as well as levels of prostaglandin E₂ and TXA₂ in diabetic renal cortexes were increased as compared to those of non-diabetic conditions. These results thus demonstrate that possibly due to up-regulated cPLA₂ and COX-2 that lead to increased prostanoid syntheses in diabetic renal cortexes, TP^-/- alleviates DN development. In addition, our results suggest that such an effect of TP^-/- might be related to the suppression of TGF-β1 up-regulation that is commonly associated with the disease condition.

Altered cyclooxygenase-1 and enhanced thromboxane receptor activities underlie attenuated endothelial dilatory capacity of omental arteries in obesity

AIMS

Obesity is a risk factor for endothelial dysfunction, the severity of which is likely to vary depending on extent and impact of adiposity on the vasculature. This study investigates the roles of cyclooxygenase isoforms and thromboxane receptor activities in the differential endothelial dilatory capacities of arteries derived from omental and subcutaneous adipose tissues in obesity.

MAIN METHODS

Small arteries were isolated from omental and subcutaneous adipose tissues obtained from consented morbidly obese patients (n = 65, BMI 45 ± 6 kg m^-2 [Mean ± SD]) undergoing bariatric surgery. Relaxation to acetylcholine was studied by wire myography in the absence or presence of indomethacin (10 μM, cyclooxygenase inhibitor), FR122047 (1 μM, cyclooxygenase-1 inhibitor), Celecoxib (4 μM, cyclooxygenase-2 inhibitor), Nω-Nitro-L-arginine methyl ester (L-NAME, 100 μM, nitric oxide synthase inhibitor) or combination of apamin (0.5 μM) and charybdotoxin (0.1 μM) that together inhibit endothelium-derived hyperpolarizing factor (EDHF). Contractions to U46619 (thromboxane A₂ mimetic) were also studied.

KEY FINDINGS

Acetylcholine relaxation was significantly attenuated in omental compared with subcutaneous arteries from same patients (p < 0.01). Indomethacin (p < 0.01) and FR122047 (p < 0.001) but not Celecoxib significantly improved the omental arteriolar relaxation. Cyclooxygenase-1 mRNA and U46619 contractions were both increased in omental compared with subcutaneous arteries (p < 0.05). L-NAME comparably inhibited acetylcholine relaxation in both arteries, while apamin+charybdotoxin were less effective in omental compared with subcutaneous arteries.

SIGNIFICANCE

The results show that the depot-specific reduction in endothelial dilatory capacity of omental compared with subcutaneous arteries in obesity is in large part due to altered cyclooxygenase-1 and enhanced thromboxane receptor activities, which cause EDHF deficiency.

Endothelial overexpression of endothelin-1 modulates aortic, carotid, iliac and renal arterial responses in obese mice

Endothelin-1 (ET-1) is essential for mammalian development and life, but it has also been implicated in increased cardiovascular risk under pathophysiological conditions. The aim of this study was to determine the impact of endothelial overexpression of the prepro-endothelin-1 gene on endothelium-dependent and endothelium-independent responses in the conduit and renal arteries of lean and obese mice. Obesity was induced by high-fat-diet (HFD) consumption in mice with Tie-1 promoter-driven, endothelium-specific overexpression of the prepro-endothelin-1 gene (TET^het) and in wild-type (WT) littermates on a C57BL/6N background. Isometric tension was measured in rings (with endothelium) of the aorta (A), carotid (CA) and iliac (IA) arteries as well as the main (MRA) and segmental renal (SRA) arteries; all experiments were conducted in the absence or presence of L-NAME and/or the COX inhibitor meclofenamate. The release of prostacyclin and thromboxane A2 was measured by ELISA. In the MRA, TET^het per se increased contractions to endothelin-1, but the response was decreased in SRA in response to serotonin; there were also improved relaxations to acetylcholine but not insulin in the SRA in the presence of L-NAME. HFD per se augmented the contractions to endothelin-1 (MRA) and to the thromboxane prostanoid (TP) receptor agonist U46619 (CA, MRA) as well as facilitated relaxations to isoproterenol (A). The combination of HFD and TET^het overexpression increased the contractions of MRA and SRA to vasoconstrictors but not in the presence of meclofenamate; this combination also augmented further relaxations to isoproterenol in the A. Contractions to endothelin-1 in the IA were prevented by endothelin-A receptor antagonist BQ-123 but only attenuated in obese mice by BQ-788. The COX-1 inhibitor FR122047 abolished the contractions of CA to acetylcholine. The release of prostacyclin during the latter condition was augmented in samples from obese TET^het mice and abolished by FR122047. These findings suggest that endothelial TET^het overexpression in lean animals has minimal effects on vascular responsiveness. However, if comorbid with obesity, endothelin-1-modulated, prostanoid-mediated renal arterial dysfunction becomes apparent.

Endothelial dysfunction and vascular disease - a 30th anniversary update

The endothelium can evoke relaxations of the underlying vascular smooth muscle, by releasing vasodilator substances. The best-characterized endothelium-derived relaxing factor (EDRF) is nitric oxide (NO) which activates soluble guanylyl cyclase in the vascular smooth muscle cells, with the production of cyclic guanosine monophosphate (cGMP) initiating relaxation. The endothelial cells also evoke hyperpolarization of the cell membrane of vascular smooth muscle (endothelium-dependent hyperpolarizations, EDH-mediated responses). As regards the latter, hydrogen peroxide (H₂ O₂ ) now appears to play a dominant role. Endothelium-dependent relaxations involve both pertussis toxin-sensitive G_i (e.g. responses to α₂ -adrenergic agonists, serotonin, and thrombin) and pertussis toxin-insensitive G_q (e.g. adenosine diphosphate and bradykinin) coupling proteins. New stimulators (e.g. insulin, adiponectin) of the release of EDRFs have emerged. In recent years, evidence has also accumulated, confirming that the release of NO by the endothelial cell can chronically be upregulated (e.g. by oestrogens, exercise and dietary factors) and downregulated (e.g. oxidative stress, smoking, pollution and oxidized low-density lipoproteins) and that it is reduced with ageing and in the course of vascular disease (e.g. diabetes and hypertension). Arteries covered with regenerated endothelium (e.g. following angioplasty) selectively lose the pertussis toxin-sensitive pathway for NO release which favours vasospasm, thrombosis, penetration of macrophages, cellular growth and the inflammatory reaction leading to atherosclerosis. In addition to the release of NO (and EDH, in particular those due to H₂ O₂ ), endothelial cells also can evoke contraction of the underlying vascular smooth muscle cells by releasing endothelium-derived contracting factors. Recent evidence confirms that most endothelium-dependent acute increases in contractile force are due to the formation of vasoconstrictor prostanoids (endoperoxides and prostacyclin) which activate TP receptors of the vascular smooth muscle cells and that prostacyclin plays a key role in such responses. Endothelium-dependent contractions are exacerbated when the production of nitric oxide is impaired (e.g. by oxidative stress, ageing, spontaneous hypertension and diabetes). They contribute to the blunting of endothelium-dependent vasodilatations in aged subjects and essential hypertensive and diabetic patients. In addition, recent data confirm that the release of endothelin-1 can contribute to endothelial dysfunction and that the peptide appears to be an important contributor to vascular dysfunction. Finally, it has become clear that nitric oxide itself, under certain conditions (e.g. hypoxia), can cause biased activation of soluble guanylyl cyclase leading to the production of cyclic inosine monophosphate (cIMP) rather than cGMP and hence causes contraction rather than relaxation of the underlying vascular smooth muscle.

Constrictor prostanoids and uridine adenosine tetraphosphate: vascular mediators and therapeutic targets in hypertension and diabetes

Vascular dysfunction plays a pivotal role in the development of systemic complications associated with arterial hypertension and diabetes. The endothelium, or more specifically, various factors derived from endothelial cells tightly regulate vascular function, including vascular tone. In physiological conditions, there is a balance between endothelium-derived factors, that is, relaxing factors (endothelium-derived relaxing factors; EDRFs) and contracting factors (endothelium-derived contracting factors; EDCFs), which mediate vascular homeostasis. However, in disease states, such as diabetes and arterial hypertension, there is an imbalance between EDRF and EDCF, with a reduction of EDRF signalling and an increase of EDCF signalling. Among EDCFs, COX-derived vasoconstrictor prostanoids play an important role in the development of vascular dysfunction associated with hypertension and diabetes. Moreover, uridine adenosine tetraphosphate (Up4 A), identified as an EDCF in 2005, also modulates vascular function. However, the role of Up4 A in hypertension- and diabetes-associated vascular dysfunction is unclear. In the present review, we focused on experimental and clinical evidence that implicate these two EDCFs (vasoconstrictor prostanoids and Up4 A) in vascular dysfunction associated with hypertension and diabetes.

Involvement of neutral sphingomyelinase in the angiotensin II signaling pathway

The possibility that angiotensin II (ANG II) exerts its effects through the activation of neutral sphingomyelinase (nSMase) has not been tested in kidneys. The results of the present study provide evidence for the activity and expression of nSMase in rat kidneys. In isolated perfused rat kidney, ANG II-induced renal vasoconstriction was inhibited by GW4869, an inhibitor of nSMase. We used nSMase for investigating the signal transduction downstream of ceramide. nSMase constricted the renal vasculature. An inhibitor of ceramidase (CDase), N-oleoylethanolamine (OEA), enhanced either ANG II- or nSMase-induced renal vasoconstriction. To demonstrate the interaction between the nSMase and cytosolic phospholipase A2 (cPLA2) signal transduction pathways, we evaluated the response to nSMase in the presence and absence of inhibitors of arachidonic acid (AA) metabolism: arachidonyl trifluoromethyl ketone (AACOCF3), an inhibitor of cPLA2; 5,8,11,14-eicosatetraynoic acid (ETYA), an inhibitor of all AA pathways; indomethacin, an inhibitor of cyclooxygenase (COX); furegrelate, a thromboxane A2 (TxA2)-synthase inhibitor; and SQ29548 , a TxA2-receptor antagonist. In these experiments, the nSMase-induced renal vasoconstriction decreased. ANG II or nSMase was associated with an increase in the release of thromboxane B2 (TxB2) in the renal perfusate of isolated perfused rat kidney. In addition, the coexpression of the ceramide with cPLA2, was found in the smooth muscle layer of intrarenal vessels. Our results suggest that ANG II stimulates ceramide formation via the activation of nSMase; thus ceramide may indirectly regulate vasoactive processes that modulate the activity of cPLA2 and the release of TxA2.

Calcitriol restores renovascular function in estrogen-deficient rats through downregulation of cyclooxygenase-2 and the thromboxane-prostanoid receptor

Cardiovascular risks increase in postmenopausal women. While vitamin D is supplemented for osteoporosis, it is not known whether it protects renal arterial function during estrogen deficiency. Here we measured changes in renovascular reactivity induced by ovariectomy in rats and examined whether calcitriol, the most active form of vitamin D, was able to correct such changes. The impairment of endothelium-dependent relaxation in renal arteries from ovariectomized rats was effectively reversed by long-term calcitriol treatment. It was also corrected by acute exposure to cyclooxygenase-2 (COX-2) inhibitors and a thromboxane-prostanoid receptor antagonist, respectively. Calcitriol normalized the overexpression of COX-2 and thromboxane-prostanoid receptors in intralobal renal artery segments and aortic endothelial cells isolated from ovariectomized rats. In vitro exposure of the arterial segments to calcitriol for 12 h improved relaxation and downregulated thromboxane-prostanoid receptors. The attenuated nitric oxide production in ovariectomized rat aortic endothelial cells was restored following a 12-h treatment with calcitriol, COX-2 inhibition, or thromboxane-prostanoid receptor antagonism. Thus, impaired endothelium-dependent renal artery relaxation in ovariectomized rats is mediated largely through increased activity and expression of COX-2 and the thromboxane-prostanoid receptor. Calcitriol restores endothelial function through downregulating both signaling proteins during estrogen deficiency.

Daily administration of the TP receptor antagonist terutroban improved endothelial function in high-cardiovascular-risk patients with atherosclerosis

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

• Terutroban is a selective TP receptor antagonist, i.e. a specific antagonist of the thromboxane A(2) and prostaglandin endoperoxide receptors, shown to improve endothelial function after a single administration in patients with coronary artery disease.

WHAT THIS STUDY ADDS

• This randomized, double-blind, placebo-controlled trial demonstrates that repeated-dose terutroban for 15 days improves endothelial function and inhibits thromboxane A(2) -induced platelet aggregation in high-cardiovascular-risk patients taking 300 mg of aspirin per day. Terutroban may prove useful for preventing cardiovascular events in such patients.

AIMS

The specific TP receptor antagonist terutroban improves endothelial function after a single dose in patients with coronary artery disease. Our aim was to evaluate the effects and dose dependency of repeated-dose terutroban on endothelial function and platelet aggregation in high-cardiovascular-risk patients with carotid atherosclerosis.

METHODS

We randomly allocated 48 patients taking 300 mg aspirin per day to placebo or to one of three terutroban dosages (2.5, 5 or 10 mg) for 15 days in a double-blind study. Flow-mediated vasodilatation was evaluated before and 2 h after the first oral dose on day 0 and 2 h after the last oral dose on day 14.

RESULTS

On day 0 and day 14, all three terutroban dosages improved flow-mediated vasodilatation and abolished platelet aggregation induced by the TP receptor agonist U46619, without changing the aggregation response to ADP or collagen.

CONCLUSION

Terutroban, by chronically improving endothelium-dependent vasodilatation and inhibiting platelet aggregation, may prove useful for preventing cardiovascular events in high-risk patients.

International Union of Basic and Clinical Pharmacology. LXXXIII: classification of prostanoid receptors, updating 15 years of progress

It is now more than 15 years since the molecular structures of the major prostanoid receptors were elucidated. Since then, substantial progress has been achieved with respect to distribution and function, signal transduction mechanisms, and the design of agonists and antagonists (http://www.iuphar-db.org/DATABASE/FamilyIntroductionForward?familyId=58). This review systematically details these advances. More recent developments in prostanoid receptor research are included. The DP(2) receptor, also termed CRTH2, has little structural resemblance to DP(1) and other receptors described in the original prostanoid receptor classification. DP(2) receptors are more closely related to chemoattractant receptors. Prostanoid receptors have also been found to heterodimerize with other prostanoid receptor subtypes and nonprostanoids. This may extend signal transduction pathways and create new ligand recognition sites: prostacyclin/thromboxane A(2) heterodimeric receptors for 8-epi-prostaglandin E(2), wild-type/alternative (alt4) heterodimers for the prostaglandin FP receptor for bimatoprost and the prostamides. It is anticipated that the 15 years of research progress described herein will lead to novel therapeutic entities.

Telmisartan inhibits vasoconstriction via PPARγ-dependent expression and activation of endothelial nitric oxide synthase

AIMS

Telmisartan activates peroxisome proliferator-activated receptor-γ (PPARγ) in addition to serving as an angiotensin II type 1 receptor (AT(1)R) blocker. The PPARγ activity of telmisartan on resistance arteries has remained largely unknown. The present study investigated the hypothesis that telmisartan inhibited vascular tension in mouse mesenteric resistance arteries, which was attributed to an increased nitric oxide (NO) production through the PPARγ-dependent augmentation of expression and activity of endothelial nitric oxide synthase (eNOS).

METHODS AND RESULTS

Second-order mesenteric arteries were isolated from male C57BL/6J, eNOS knockout and PPARγ knockout mice and changes in vascular tension were determined by isometric force measurement with a myograph. Expression and activation of relevant proteins were analysed by Western blotting. Real-time NO production was measured by confocal microscopy using the dye DAF. Telmisartan inhibited 9,11-dideoxy-11α,9α-epoxymethanoprostaglandin F(2α) (U46619)- or endothelin-1-induced contractions. An NOS inhibitor, N(G)-nitro-L-arginine methyl ester (l-NAME), or an inhibitor of soluble guanylate cyclase, 1H-[1,2,4]-oxadizolo[4,3-a]quinoxalin-1-one (ODQ), prevented telmisartan-induced inhibition of U46619 contractions. A PPARγ antagonist, GW9662, abolished telmisartan-induced inhibition. Likewise, the PPARγ antagonist rosiglitazone attenuated U46619-induced contractions. The effects of telmisartan and rosiglitazone were prevented by actinomycin-D, a transcription inhibitor. In contrast, losartan, olmesartan, and irbesartan did not inhibit contractions. The inhibition was absent in mesenteric arteries from eNOS knockout or PPARγ knockout mice. Telmisartan augmented eNOS expression, phosphorylation, and NO production, which were reversed by the co-treatment with GW9662.

CONCLUSIONS

The present results suggest that telmisartan-induced inhibition of vasoconstriction in resistance arteries is mediated through a PPARγ-dependent increase in eNOS expression and activity that is unrelated to AT₁R blockade.

The thromboxane/endoperoxide receptor (TP): the common villain

The stimulation of thromboxane/endoperoxide receptors (TP) elicits diverse physiological/pathophysiological reactions, including platelet aggregation and contraction of vascular smooth muscle. Furthermore, the activation of endothelial TP promotes the expression of adhesion molecules and favors adhesion and infiltration of monocytes/macrophages. In various cardiovascular diseases, endothelial dysfunction is predominantly the result of the release of endothelium-derived contracting factors that counteract the vasodilator effect of nitric oxide produced by the endothelial nitric oxide synthase. Endothelium-dependent contractions involve the activation of cyclooxygenases, the production of reactive oxygen species along with that of endothelium-derived contracting factors, which diffuse toward the vascular smooth muscle cells and activate their TP. TP antagonists curtail the endothelial dysfunction in diseases such as hypertension and diabetes, are potent antithrombotic agents, and reduce vascular inflammation. Therefore, TP antagonists, because of this triple activity, may have a unique potential for the treatment of cardiovascular disorders.

Distinct roles for basal and induced COX-2 in podocyte injury

Transgenic mice that overexpress cyclooxygenase-2 (COX-2) selectively in podocytes are more susceptible to glomerular injury by adriamycin and puromycin (PAN). To investigate the potential roles of COX-2 metabolites, we studied mice with selective deletion of prostanoid receptors and generated conditionally immortalized podocyte lines from mice with either COX-2 deletion or overexpression. Podocytes that overexpressed COX-2 were virtually indistinguishable from wild-type podocytes but were significantly more sensitive to PAN-induced injury, produced more prostaglandin E(2) and thromboxane B(2), and had greater expression of prostaglandin E(2) receptor subtype 4 (EP(4)) and thromboxane receptor (TP). Treatment of COX-2-overexpressing podocytes with a TP antagonist reduced apoptosis, but treatment with an EP(4) antagonist did not. In contrast, podocytes from COX-2-knockout mice exhibited increased apoptosis, markedly decreased cell adhesion, and prominent stress fibers. In vivo, selective deletion of podocyte EP(4) did not alter the increased sensitivity to adriamycin-induced injury observed in mice overexpressing podocyte COX-2. In contrast, genetic deletion of TP in these mice prevented adriamycin-induced injury, with attenuated albuminuria and foot process effacement. These results suggest that basal COX-2 may be important for podocyte survival, but overexpression of podocyte COX-2 increases susceptibility to podocyte injury, which is mediated, in part, by activation of the thromboxane receptor.

Endothelial dysfunction and vascular disease

The endothelium can evoke relaxations (dilatations) of the underlying vascular smooth muscle, by releasing vasodilator substances. The best characterized endothelium-derived relaxing factor (EDRF) is nitric oxide (NO). The endothelial cells also evoke hyperpolarization of the cell membrane of vascular smooth muscle (endothelium-dependent hyperpolarizations, EDHF-mediated responses). Endothelium-dependent relaxations involve both pertussis toxin-sensitive G(i) (e.g. responses to serotonin and thrombin) and pertussis toxin-insensitive G(q) (e.g. adenosine diphosphate and bradykinin) coupling proteins. The release of NO by the endothelial cell can be up-regulated (e.g. by oestrogens, exercise and dietary factors) and down-regulated (e.g. oxidative stress, smoking and oxidized low-density lipoproteins). It is reduced in the course of vascular disease (e.g. diabetes and hypertension). Arteries covered with regenerated endothelium (e.g. following angioplasty) selectively loose the pertussis toxin-sensitive pathway for NO release which favours vasospasm, thrombosis, penetration of macrophages, cellular growth and the inflammatory reaction leading to atherosclerosis. In addition to the release of NO (and causing endothelium-dependent hyperpolarizations), endothelial cells also can evoke contraction (constriction) of the underlying vascular smooth muscle cells by releasing endothelium-derived contracting factor (EDCF). Most endothelium-dependent acute increases in contractile force are due to the formation of vasoconstrictor prostanoids (endoperoxides and prostacyclin) which activate TP receptors of the vascular smooth muscle cells. EDCF-mediated responses are exacerbated when the production of NO is impaired (e.g. by oxidative stress, ageing, spontaneous hypertension and diabetes). They contribute to the blunting of endothelium-dependent vasodilatations in aged subjects and essential hypertensive patients.

Endothelium-dependent contractions in SHR: a tale of prostanoid TP and IP receptors

In the aorta of spontaneously hypertensive rats (SHR), the endothelial dysfunction is due to the release of endothelium-derived contracting factors (EDCFs) that counteract the vasodilator effect of nitric oxide, with no or minor alteration of its production. The endothelium-dependent contractions elicited by acetylcholine (ACh) involve an increase in endothelial [Ca(2+)](i), the production of reactive oxygen species, the activation of endothelial cyclooxygenase-1, the diffusion of EDCF and the subsequent stimulation of smooth muscle cell TP receptors. The EDCFs released by ACh have been identified as PGH(2) and paradoxically prostacyclin. Prostacyclin generally acts as an endothelium-derived vasodilator, which, by stimulating IP receptors, produces hyperpolarization and relaxation of the smooth muscle and inhibits platelet aggregation. In the aorta of SHR and Wistar-Kyoto rats, prostacyclin is the principal metabolite of arachidonic acid released by ACh. However, in SHR aorta, prostacyclin does not produce relaxations but activates the TP receptors on vascular smooth muscle cells and produces contraction. The IP receptor is not functional in the aortic smooth muscle cells of SHR as early as 12 weeks of age, but its activity is not reduced in platelets. Therefore, prostacyclin in the rule protects the vascular wall, but in the SHR aorta it can contribute to endothelial dysfunction. Whether or not prostacyclin plays a detrimental role as an EDCF in other animal models or in human remains to be demonstrated. Nevertheless, because EDCFs converge to activate TP receptors, selective antagonists of this receptor, by preventing endothelium-dependent contractions, curtail the endothelial dysfunction in diseases such as hypertension and diabetes.

Endothelium-dependent contractions: when a good guy turns bad!

Endothelial cells can induce contractions of the underlying vascular smooth muscle by generating vasoconstrictor prostanoids (endothelium-dependent contracting factor; EDCF). The endothelial COX-1 isoform of cyclooxygenase appears to play the dominant role in the phenomenon. Its activation requires an increase in intracellular Ca(2+) concentration. The production of EDCF is inhibited acutely and chronically by nitric oxide (NO), and possibly by endothelium-dependent hyperpolarizing factor (EDHF). The main prostanoids involved in endothelium-dependent contractions appear to be endoperoxides (PGH(2)) and prostacyclin, which activate thromboxane-prostanoid (TP) receptors of the vascular smooth muscle cells. Oxygen-derived free radicals can facilitate the production and/or the action of EDCF. Endothelium-dependent contractions are exacerbated by ageing, obesity, hypertension and diabetes, and thus are likely to contribute to the endothelial dysfunction observed in older people and in essential hypertensive patients.