Regulation of P-450 4A activity in the glomerulus of the rat

Conflicting roles of 20-HETE in hypertension and renal end organ damage

20-HETE is a cytochrome P450-derived metabolite of arachidonic acid that has both pro- and anti-hypertensive actions that result from modulation of vascular and kidney function. In the vasculature, 20-HETE sensitizes vascular smooth muscle cells to constrictor stimuli and increases myogenic tone. By promoting smooth muscle cell migration and proliferation, as well as by acting on the vascular endothelium to cause endothelial dysfunction, angiotensin converting enzyme (ACE) expression, and inflammation, 20-HETE contributes to adverse vascular remodeling and increased blood pressure. A G protein-coupled receptor was recently identified as the effector for the vascular actions of 20-HETE. In addition, evidence suggests that 20-HETE contributes to hypertension via positive regulation of the renin-angiotensin-aldosterone system, as well as by causing renal fibrosis. On the other hand, 20-HETE exerts anti-hypertensive actions by inhibiting sodium reabsorption by the kidney in both the proximal tubule and thick ascending limb of Henle. This review discusses the pro- and anti-hypertensive roles of 20-HETE in the pathogenesis of hypertension-associated renal disease, the association of gene polymorphisms of cytochrome P450 enzymes with the development of hypertension and renal end organ damage in humans, and 20-HETE related pharmaceutical agents.

Clinical Implications of 20-Hydroxyeicosatetraenoic Acid in the Kidney, Liver, Lung and Brain: An Emerging Therapeutic Target

Cytochrome P450-mediated metabolism of arachidonic acid (AA) is an important pathway for the formation of eicosanoids. The ω-hydroxylation of AA generates significant levels of 20-hydroxyeicosatetraenoic acid (20-HETE) in various tissues. In the current review, we discussed the role of 20-HETE in the kidney, liver, lung, and brain during physiological and pathophysiological states. Moreover, we discussed the role of 20-HETE in tumor formation, metabolic syndrome and diabetes. In the kidney, 20-HETE is involved in modulation of preglomerular vascular tone and tubular ion transport. Furthermore, 20-HETE is involved in renal ischemia/reperfusion (I/R) injury and polycystic kidney diseases. The role of 20-HETE in the liver is not clearly understood although it represents 50%-75% of liver CYP-dependent AA metabolism, and it is associated with liver cirrhotic ascites. In the respiratory system, 20-HETE plays a role in pulmonary cell survival, pulmonary vascular tone and tone of the airways. As for the brain, 20-HETE is involved in cerebral I/R injury. Moreover, 20-HETE has angiogenic and mitogenic properties and thus helps in tumor promotion. Several inhibitors and inducers of the synthesis of 20-HETE as well as 20-HETE analogues and antagonists are recently available and could be promising therapeutic options for the treatment of many disease states in the future.

Molecular mechanisms and cell signaling of 20-hydroxyeicosatetraenoic acid in vascular pathophysiology

Cytochrome P450s enzymes catalyze the metabolism of arachidonic acid to epoxyeicosatrienoic acids (EETs), dihydroxyeicosatetraenoic acid and hydroxyeicosatetraeonic acid (HETEs). 20-HETE is a vasoconstrictor that depolarizes vascular smooth muscle cells by blocking K+ channels. EETs serve as endothelial derived hyperpolarizing factors. Inhibition of the formation of 20-HETE impairs the myogenic response and autoregulation of renal and cerebral blood flow. Changes in the formation of EETs and 20-HETE have been reported in hypertension and drugs that target these pathways alter blood pressure in animal models. Sequence variants in CYP4A11 and CYP4F2 that produce 20-HETE, UDP-glucuronosyl transferase involved in the biotransformation of 20-HETE and soluble epoxide hydrolase that inactivates EETs are associated with hypertension in human studies. 20-HETE contributes to the regulation of vascular hypertrophy, restenosis, angiogenesis and inflammation. It also promotes endothelial dysfunction and contributes to cerebral vasospasm and ischemia-reperfusion injury in the brain, kidney and heart. This review will focus on the role of 20-HETE in vascular dysfunction, inflammation, ischemic and hemorrhagic stroke and cardiac and renal ischemia reperfusion injury.

High salt diet exacerbates vascular contraction in the absence of adenosine A₂A receptor

High salt (4% NaCl, HS) diet modulates adenosine-induced vascular response through adenosine A(2A) receptor (A(2A)AR). Evidence suggests that A(2A)AR stimulates cyp450-epoxygenases, leading to epoxyeicosatrienoic acids (EETs) generation. The aim of this study was to understand the vascular reactivity to HS and underlying signaling mechanism in the presence or absence of A(2A)AR. Therefore, we hypothesized that HS enhances adenosine-induced relaxation through EETs in A(2A)AR⁺/⁺, but exaggerates contraction in A(2A)AR⁻/⁻. Organ bath and Western blot experiments were conducted in HS and normal salt (NS, 0.18% NaCl)-fed A(2A)AR⁺/⁺ and A(2A)AR⁻/⁻ mice aorta. HS produced concentration-dependent relaxation to non-selective adenosine analog, NECA in A(2A)AR⁺/⁺, whereas contraction was observed in A(2A)AR⁻/⁻ mice and this was attenuated by A₁AR antagonist (DPCPX). CGS 21680 (selective A(2A)AR agonist) enhanced relaxation in HS-A(2A)AR⁺/⁺ versus NS-A(2A)AR⁺/⁺, which was blocked by EETs antagonist (14,15-EEZE). Compared with NS, HS significantly upregulated the expression of vasodilators A(2A)AR and cyp2c29, whereas vasoconstrictors A₁AR and cyp4a in A(2A)AR⁺/⁺ were downregulated. In A(2A)AR⁻/⁻ mice, however, HS significantly downregulated the expression of cyp2c29, whereas A₁AR and cyp4a were upregulated compared with A(2A)AR⁺/⁺ mice. Hence, our data suggest that in A(2A)AR⁺/⁺, HS enhances A(2A)AR-induced relaxation through increased cyp-expoxygenases-derived EETs and decreased A₁AR levels, whereas in A(2A)AR⁻/⁻, HS exaggerates contraction through decreased cyp-epoxygenases and increased A₁AR levels.

Impaired myogenic response and autoregulation of cerebral blood flow is rescued in CYP4A1 transgenic Dahl salt-sensitive rat

We have reported that a reduction in renal production of 20-HETE contributes to development of hypertension in Dahl salt-sensitive (SS) rats. The present study examined whether 20-HETE production is also reduced in the cerebral vasculature of SS rats and whether this impairs the myogenic response and autoregulation of cerebral blood flow (CBF). The production of 20-HETE, the myogenic response of middle cerebral arteries (MCA), and autoregulation of CBF were compared in SS, SS-5(BN) rats and a newly generated CYP4A1 transgenic rat. 20-HETE production was 6-fold higher in cerebral arteries of CYP4A1 and SS-5(BN) than in SS rats. The diameter of the MCA decreased to 70 ± 3% to 65 ± 6% in CYP4A1 and SS-5(BN) rats when pressure was increased from 40 to 140 mmHg. In contrast, the myogenic response of MCA isolated from SS rats did not constrict. Administration of a 20-HETE synthesis inhibitor, HET0016, abolished the myogenic response of MCA in CYP4A1 and SS-5(BN) rats but had no effect in SS rats. Autoregulation of CBF was impaired in SS rats compared with CYP4A1 and SS-5(BN) rats. Blood-brain barrier leakage was 5-fold higher in the brain of SS rats than in SS-5(BN) and SS.CYP4A1 rats. These findings indicate that a genetic deficiency in the formation of 20-HETE contributes to an impaired myogenic response in MCA and autoregulation of CBF in SS rats and this may contribute to vascular remodeling and cerebral injury following the onset of hypertension.

20-HETE and blood pressure regulation: clinical implications

20-Hydroxy-5, 8, 11, 14-eicosatetraenoic acid (20-HETE) is a cytochrome P450 (CYP)-derived omega-hydroxylation metabolite of arachidonic acid. 20-HETE has been shown to play a complex role in blood pressure regulation. In the kidney tubules, 20-HETE inhibits sodium reabsorption and promotes natriuresis, thus, contributing to antihypertensive mechanisms. In contrast, in the microvasculature, 20-HETE has been shown to play a pressor role by sensitizing smooth muscle cells to constrictor stimuli and increasing myogenic tone, and by acting on the endothelium to further promote endothelial dysfunction and endothelial activation. In addition, 20-HETE induces endothelial angiotensin-converting enzyme, thus, setting forth a potential feed forward prohypertensive mechanism by stimulating the renin-angiotensin-aldosterone system. With the advancement of gene sequencing technology, numerous polymorphisms in the regulatory coding and noncoding regions of 20-HETE-producing enzymes, CYP4A11 and CYP4F2, have been associated with hypertension. This in-depth review article discusses the biosynthesis and function of 20-HETE in the cardiovascular system, the pharmacological agents that affect 20-HETE action, and polymorphisms of CYP enzymes that produce 20-HETE and are associated with systemic hypertension in humans.

20-Hydroxyeicosatetraenoic acid contributes to the inhibition of K+ channel activity and vasoconstrictor response to angiotensin II in rat renal microvessels

The present study examined whether 20-hydroxyeicosatetraenoic acid (HETE) contributes to the vasoconstrictor effect of angiotensin II (ANG II) in renal microvessels by preventing activation of the large conductance Ca²⁺-activated K⁺ channel (K_Ca) in vascular smooth muscle (VSM) cells. ANG II increased the production of 20-HETE in rat renal microvessels. This response was attenuated by the 20-HETE synthesis inhibitors, 17-ODYA and HET0016, a phospholipase A₂ inhibitor AACOF₃, and the AT₁ receptor blocker, Losartan, but not by the AT₂ receptor blocker, PD123319. ANG II (10^-11 to 10^-6 M) dose-dependently decreased the diameter of renal microvessels by 41 ± 5%. This effect was blocked by 17-ODYA. ANG II (10^-7 M) did not alter K_Ca channel activity recorded from cell-attached patches on renal VSM cells under control conditions. However, it did reduce the NPo of the K_Ca channel by 93.4 ± 3.1% after the channels were activated by increasing intracellular calcium levels with ionomycin. The inhibitory effect of ANG II on K_Ca channel activity in the presence of ionomycin was attenuated by 17-ODYA, AACOF₃, and the phospholipase C (PLC) inhibitor U-73122. ANG II induced a peak followed by a steady-state increase in intracellular calcium concentration in renal VSM cells. 17-ODYA (10^-5 M) had no effect on the peak response, but it blocked the steady-state increase. These results indicate that ANG II stimulates the formation of 20-HETE in rat renal microvessels via the AT₁ receptor activation and that 20-HETE contributes to the vasoconstrictor response to ANG II by blocking activation of K_Ca channel and facilitating calcium entry.

Induction of angiotensin-converting enzyme and activation of the renin-angiotensin system contribute to 20-hydroxyeicosatetraenoic acid-mediated endothelial dysfunction

OBJECTIVE

20-hydroxyeicosatetraenoic acid (20-HETE) promotes endothelial dysfunction by uncoupling endothelial NO synthase, stimulating O(2)(-) production, and reducing NO bioavailability. Moreover, 20-HETE-dependent vascular dysfunction and hypertension are associated with upregulation of the renin-angiotensin system This study was undertaken to examine the contribution of renin-angiotensin system to 20-HETE actions in the vascular endothelium.

METHODS AND RESULTS

In endothelial cells, 20-HETE induced angiotensin-converting enzyme (ACE) mRNA levels and increased ACE protein and activity by 2- to 3-fold; these effects were negated with addition of the 20-HETE antagonist, 20-hydroxyeicosa-6(Z),15(Z)-dienoic acid (20 HEDE). 20-HETE induced ACE expression was protein kinase C independent and epidermal growth factor receptor tyrosine kinase and IκB kinase β dependent. ACE short interfering RNA abolished 20-HETE-mediated inhibition of NO production and stimulation of O(2)(-) generation, whereas angiotensin II type 1 receptor short interfering RNA attenuated these effects by 40%. 20-HETE-stimulated O(2)(-) production was negated by 20-HEDE and was attenuated by lisinopril and losartan. Importantly, 20-HETE-mediated impairment of acetylcholine-induced relaxation in rat renal interlobar arteries was also attenuated by lisinopril and losartan.

CONCLUSIONS

These results indicate that ACE and angiotensin II type 1 receptor activation contribute to 20-HETE-mediated endothelial cell and vascular dysfunction and further enforce the notion that excessive production of 20-HETE within the vasculature leads to hypertension via mechanisms that include the induction of endothelial ACE, thus, perpetuating an increase in vascular angiotensin which, together with 20-HETE, promotes vascular dysfunction.

20-hydroxyeicosatetraeonic acid: a new target for the treatment of hypertension

Arachidonic acid is metabolized by enzymes of the CYP4A and 4F families to 20-hydroxyeicosatetraeonic acid (20-HETE), which plays an important role in the regulation of renal function, vascular tone, and the long-term control of arterial pressure. In the vasculature, 20-HETE is a potent vasoconstrictor, and upregulation of the production of this compound contributes to the elevation in oxidative stress and endothelial dysfunction and the increase in peripheral vascular resistance associated with some forms of hypertension. In kidney, 20-HETE inhibits Na transport in the proximal tubule and thick ascending loop of Henle, and deficiencies in the renal formation of 20-HETE contributes to sodium retention and development of some salt-sensitive forms of hypertension. 20-HETE also has renoprotective actions and opposes the effects of transforming growth factor β to promote proteinuria and renal end organ damage in hypertension. Several new inhibitors of the synthesis of 20-HETE and 20-HETE agonists and antagonists have recently been developed. These compounds along with peroxisome proliferator-activated receptor-α agonists that induce the renal formation of 20-HETE seem to have promise as antihypertensive agents. This review summarizes the rationale for the development of drugs that target the 20-HETE pathway for the treatment of hypertension and associated cardiovascular complications.

Modulation by salt intake of the vascular response mediated through adenosine A(2A) receptor: role of CYP epoxygenase and soluble epoxide hydrolase

High-salt intake can change the effect of adenosine on arterial tone in mice. The aim of this study was to clarify the mechanism by which this occurs. Using aortas from mice fed a 4% NaCl (HS) or 0.45% NaCl (NS) diet for 4-5 wks, concentration-response curves for ACh, 5'-N-ethylcarboxamidoadenosine (NECA; adenosine analog) and 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride hydrate [CGS-21680; A(2A) adenosine receptor (A(2A) AR) agonist] were obtained with N(omega)-nitro-L-arginine methyl ester (L-NAME; nitric oxide inhibitor, 10(-4) M), methylsulfonyl-propargyloxyphenylhexanamide [MS-PPOH; a CYP (cytochrome P-450) epoxygenase blocker, 10(-5) M including CYP2J2], 12-(3-adamantan-1-yl-ureido)dodecanoic acid [AUDA; soluble epoxide hydrolase (sEH) blocker, 10(-5) M], dibromo-dodecenyl-methylsulfimide [DDMS; CYP omega-hydroxylase (CYP4A blocker), 10(-5) M], glibenclamide (K(ATP) channel blocker; 10(-5) M) and 5-hydroxydecanoate (5-HD; mitochondrial-K(ATP) channel blocker, 10(-4) M). HS dose response to ACh (10(-7) - 10(-5) M) was not different from NS (P > 0.05). Relaxation to 10(-6) M NECA was greater in the HS group (28.4 +/- 3.9%) than in the NS group (4.1 +/- 2.3%). Relaxation to 10(-6) M CGS-21680 was also greater in HS (27.9 +/- 4.5%) than in NS (4.9 +/- 2.2%). L-NAME was able to block the dose response of ACh (10(-7) - 10(-5) M) equally in both HS and NS (P > 0.05), whereas L-NAME did not block CGS-21680-induced response in HS. In HS the CGS-21680 response was greatly reduced by MS-PPOH (to 4.7 +/- 2.0%) and 5-HD (to 8.9 +/- 2.2%), and also abolished by glibenclamide (-1.0 +/- 5.9%). In NS, the CGS-21680 response was increased by AUDA (to 26.3 +/- 3.4%) and DDMS (to 27.2 +/- 3.0%). Compared with NS, HS vessels showed increased CYP2J2 and A(2A) AR expression (46 and 74% higher, respectively) but decreased sEH, CYP4A, and A(1) AR expression (75, 30, and 55% lower, respectively). These data suggest that in mice fed NS-containing diet, upregulation of arterial A(1) receptor causes vasoconstriction via increased sEH and CYP4A proteins. However, in mice fed HS-containing diet, upregulation of A(2A) receptor protein triggers vascular relaxation through ATP-sensitive (K(+)) channels via upregulation of CYP2J2 enzyme.

Glomerular 20-HETE, EETs, and TGF-beta1 in diabetic nephropathy

The early stage of diabetic nephropathy (DN) is linked to proteinuria. Transforming growth factor (TGF)-beta1 increases glomerular permeability to albumin (P(alb)), whereas 20-HETE and EETs reduce P(alb). To investigate the impact of hyperglycemia and hyperlipidemia on 20-HETE, EETs, and TGF-beta1 in the glomeruli, rats were divided into four groups: ND rats were fed a normal diet, HF rats were fed a high-fat diet, STZ rats were treated with 35 mg/kg of streptozotocin, and HF/STZ rats were fed a HF diet and treated with STZ. After 10 wk on these regimens, blood glucose, urinary albumin, serum cholesterol, serum triglyceride levels, and the kidney-to-body weight ratio were significantly elevated in STZ and HF/STZ rats compared with HF and ND rats. STZ and HF/STZ rats had histopathologic changes and abnormal renal hemodynamics. Expression of glomerular CYP4A, enzymes for 20-HETE production, was significantly decreased in STZ rats, whereas expression of glomerular CYP2C and CYP2J, enzymes for EETs production, was significantly decreased in both STZ and HF/STZ rats. Moreover, glomerular TGF-beta1 levels were significantly greater in STZ and HF/STZ rats than in HF and ND rats. Five-week treatment of STZ rats with clofibrate induced glomerular CYP4A expression and 20-HETE production, but reduced glomerular TGF-beta1 and urinary protein excretion. These results demonstrate that hyperglycemia increases TGF-beta1 but decreases 20-HETE and EETs production in the glomeruli, changes that may be important in causing glomerular damage in the early stage of DN.

High-salt diet enhances mouse aortic relaxation through adenosine A2A receptor via CYP epoxygenases

We hypothesize that A(2A) adenosine receptors (A(2A) AR) promote aortic relaxation in mice through cytochrome P450 (CYP)-epoxygenases and help to avoid salt sensitivity. Aortas from male mice maintained on a high-salt (HS; 7% NaCl) or normal-salt (NS; 0.45% NaCl) diet for 4-5 wks were used. Concentration-response curves (10(-11)-10(-5) M) for 5'-N-ethylcarboxamidoadenosine (NECA; a nonselective adenosine analog) and CGS 21680 (A(2A) AR agonist) were obtained with different antagonists including ZM 241385 (A(2A) AR antagonist; 10(-6) M), SCH 58261 (A(2A) AR antagonist; 10(-6) M), N(omega)-nitro-l-arginine methyl ester (l-NAME; endothelial nitric oxide synthase inhibitor; 10(-4) M) and inhibitors including methylsulfonyl-propargyloxyphenylhexanamide (MS-PPOH; CYP epoxygenases inhibitor; 10(-5)M), 14,15-epoxyeicosa-5(z)-enoic acid (14,15-EEZE; EET antagonist; 10(-5)M), dibromo-dodecenyl-methylsulfimide (DDMS; CYP4A inhibitor; 10(-5)M), and HET0016 (20-HETE inhibitor; 10(-5)M). At 10(-7) M of NECA, significant relaxation in HS (+22.58 +/- 3.12%) was observed compared with contraction in NS (-10.62 +/- 6.27%, P < 0.05). ZM 241385 changed the NECA response to contraction (P < 0.05) in HS. At 10(-7) M of CGS 21680, significant relaxation in HS (+32.04 +/- 3.08%) was observed compared with NS (+10.45 +/- 1.34%, P < 0.05). SCH 58261, l-NAME, MS-PPOH, and 14,15-EEZE changed the CGS 21680-induced relaxation to contraction (P < 0.05) in HS. Interestingly, DDMS and HET0016 changed CGS 21680 response to relaxation (P < 0.05) in NS; however, there was no significant difference found between DDMS, HET0016-treated HS and NS vs. nontreated HS group (P > 0.05). CYP2C29 protein was 55% and 74% upregulated in HS vs. NS (P < 0.05) mice aorta and kidney, respectively. CYP4A protein was 30.30% and 35.70% upregulated in NS vs. HS (P < 0.05) mice aorta and kidneys, respectively. A(1) AR was downregulated, whereas A(2A) AR was upregulated in HS compared with NS. These data suggest that HS may activate CYP2C29 via A(2A) AR, causing relaxation, whereas NS may contribute to the upregulation of CYP4A causing contraction.

20-HETE-mediated cytotoxicity and apoptosis in ischemic kidney epithelial cells

20-HETE, a metabolite of arachidonic acid, has been implicated as a mediator of free radical formation and tissue death following ischemia-reperfusion (IR) injury in the brain and heart. The present study examined the role of this pathway in a simulated IR renal injury model in vitro. Modified self-inactivating lentiviral vectors were generated to stably overexpress murine Cyp4a12 following transduction into LLC-PK(1) cells (LLC-Cyp4a12). We compared the survival of control and transduced LLC-PK(1) cells following 4 h of ATP depletion and 2 h of recovery in serum-free medium. ATP depletion-recovery of LLC-Cyp4a12 cells resulted in a significantly higher LDH release (P < 0.05) compared with LLC-enhanced green fluorescent protein (EGFP) cells. Treatment with the SOD mimetic MnTMPyP (100 microM) resulted in decreased cytotoxicity in LLC-Cyp4a12 cells. The selective 20-HETE inhibitor HET-0016 (10 microM) also inhibited cytotoxicity significantly (P < 0.05) in LLC-Cyp4a12 cells. Dihydroethidium fluorescence showed that superoxide levels were increased to the same degree in LLC-EGFP and LLC-Cyp4a12 cells after ATP depletion-recovery compared with control cells and that this increase was inhibited by MnTMPyP. There was a significant increase (P < 0.05) of caspase-3 cleavage, an effector protease of the apoptotic pathway, in the LLC-Cyp4a12 vs. LLC-EGFP cells (P < 0.05). This was abolished in the presence of HET-0016 (P < 0.05) or MnTMPyP (P < 0.01). These results demonstrate that 20-HETE overexpression can significantly exacerbate the cellular damage that is associated with renal IR injury and that the programmed cell death is mediated by activation of caspase-3 and is partially dependent on enhanced CYP4A generation of free radicals.

Role of endogenous CYP450 metabolites of arachidonic acid in maintaining the glomerular protein permeability barrier

This study examined the metabolism of arachidonic acid (AA) by cytochrome P-450 enzymes in isolated glomeruli and the effects of selective inhibitors of the synthesis of 20-hydroxyeicosatetraenoic acid (20-HETE) and epoxyeicosatetraenoic acids (EETs) on glomerular permeability to albumin (P(alb)). Glomeruli avidly produced 20-HETE, EETs, dihydroxyeicosatetraenoic acids (diHETEs), and HETEs when incubated with exogenous AA. N-hydroxy-N'-(4-butyl-2-methylphenyl)formamidine (HET0016; 10 microM) selectively inhibited the formation of 20-HETE by 95% and increased P(alb) from 0.00 +/- 0.08 to 0.73 +/- 0.10 (n = 43 glomeruli, 4 rats). Addition of a 20-HETE mimetic, 20-hydroxyeicosa-5(Z),14(Z)-dienoic acid (20-5,14-HEDE; 1 microM) opposed the effects of HET0016 (10 microM) to increase P(alb) (0.21 +/- 0.10, n = 36 glomeruli, 4 rats). Preincubation of glomeruli with exogenous AA to increase basal production of 20-HETE had a similar effect. We also examined the effect of an epoxygenase inhibitor, N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide (MSPPOH; 5 microM), on P(alb). MSPPOH (5 microM) significantly increased P(alb) but had no effect on the synthesis of EETs in glomeruli incubated with AA. However, MSPPOH (5 microM) selectively reduced epoxygenase activity by 50% in glomeruli incubated without added AA. Pretreatment with 8,9-EET (100 nM) attenuated the effects of MSPPOH (5 microM) on P(alb). These results indicate that glomeruli produce 20-HETE, EETs, diHETEs, and HETEs and that endogenously formed 20-HETE and EETs play an essential role in the maintenance of the glomerular permeability barrier to albumin.

Glomerular cytochrome P-450 and cyclooxygenase metabolites regulate efferent arteriole resistance

Bradykinin dilates efferent arterioles via release of efferent arteriole epoxyeicosatrienoic acids when perfused retrograde (no glomerular autacoids). However, when efferent arterioles are perfused orthograde through the glomerulus, bradykinin-induced dilatation is caused by a balance between: (1) the glomerular vasoconstrictor 20-hydroxyeicosatetraenoic acid and vasodilator prostaglandins, and (2) epoxyeicosatrienoic acids from the efferent arteriole and possibly the glomerulus. However, the role of 20-hydroxyeicosatetraenoic acid has only been studied with a cyclooxygenase inhibitor, which may artificially enhance its production by shunting arachidonic acid into the cytochrome P450 pathway. We hypothesized that in the absence of cyclooxygenase inhibition, bradykinin induces release of 20-hydroxyeicosatetraenoic acid from the glomerulus, which blunts the vasodilator effect of bradykinin; and that prostaglandins released from glomeruli in response to bradykinin are generated by cyclooxygenase-1. Rabbit efferent arterioles preconstricted with norepinephrine were perfused orthograde from the end of the afferent arteriole. Bradykinin was added to the perfusate with or without a 20-hydroxyeicosatetraenoic acid antagonist (20-HEDE), epoxyeicosatrienoic acid synthesis inhibitor (MS-PPOH), and/or cyclooxygenase-1 (SC-58560) or cyclooxygenase-2 inhibitor (NS-398). Bradykinin-dependent dilatation was enhanced by 20-HEDE but blunted by MS-PPOH. When the inhibitors were present, bradykinin-induced dilatation was abolished by blockade of cyclooxygenase-1 but not cyclooxygenase-2. We concluded that: (1) in the absence of cyclooxygenase inhibitors, bradykinin causes the release of a glomerular vasoconstrictor (20-hydroxyeicosatetraenoic acid) that antagonizes the vasodilator effect of epoxyeicosatrienoic acids released from the efferent arteriole and perhaps from the glomerulus, and (2) bradykinin-induced vasodilatation is caused by the release of epoxyeicosatrienoic acids from the efferent arteriole and glomerular metabolites of cyclooxygenase-1.

12-Hydroxyeicosatetraenoic acid participates in angiotensin II afferent arteriolar vasoconstriction by activating L-type calcium channels

The lipoxygenase (LO) metabolite, 12(S)-hydroxyeicosatetraenoic acid [12(S)-HETE], constricts renal vessels, contributes to the vascular response to angiotensin, and has been implicated in cardiovascular and renal diseases. The current studies were performed to determine if renal microvascular 12(S)-HETE production is stimulated by angiotensin and the contribution of L-type calcium channels to the vasoconstriction elicited by 12(S)-HETE. Angiotensin increased renal microvascular 12(S)-HETE production by 64%, whereas cyclooxygenase metabolite production was not altered. Renal microvessels also expressed platelet-type 12-LO and leukocyte-type 12-LO. In the juxtamedullary preparation, afferent arteriolar diameter averaged 21 +/- 1 microm and 12(S)-HETE caused a graded decrease in vessel caliber. The afferent arteriolar response to 12(S)-HETE was abolished during L-type calcium channel inhibition. Renal microvascular smooth muscle cells were studied using fluorescence microscopy. Renal myocyte [Ca2+]i averaged 93 +/- 5 nmol/l. The 12(S)-HETE (5 micromol/l) increased myocyte [Ca2+]i to a peak value of 340 +/- 55 nmol/l. The peak [Ca2+]i response following exposure to 12(S)-HETE was greatly attenuated in the absence of extracellular Ca2+ or calcium channel blockade. These results demonstrate that renal microvascular 12(S)-HETE production is increased in response to angiotensin, and activation of L-type calcium channels is an important mechanism responsible for the afferent arteriolar vasoconstriction elicited by 12(S)-HETE.

Inhibitors of 20-HETE formation promote salt-sensitive hypertension in rats

This study examined whether chronic blockade of epoxyeicosatrienoic acids (EETs) and/or 20-hydroxyeicosatetraenoic acid (20-HETE) formation promotes development of salt-sensitive hypertension. Changes in blood pressure, renal cytochrome P450 metabolism of arachidonic acid, and 20-HETE excretion in response to a high salt diet were measured in rats chronically treated with 1-aminobenzotriazole (ABT, 50 mg/kg per day) to block EETs and 20-HETE formation or N-hydroxy-N'-(4-butyl-2 methylphenyl) formamidine (HET0016, 10 mg/kg per day) that selectively reduces 20-HETE formation. ABT reduced blood pressure in rats fed a low salt (0.4% NaCl) diet, but blood pressure rose by 20 mm Hg after these rats were switched to a high salt (8% NaCl) diet for 10 days. HET0016 had no effect on blood pressure in rats fed a low salt diet; however, blood pressure rose by 18 mm Hg after the rats were fed a high salt diet. 20-HETE formation in kidney homogenates rose by 30% and epoxygenase activity doubled when rats were fed a high salt diet. Chronic treatment with ABT and HET0016 inhibited the renal formation of 20-HETE by approximately 90%. Renal epoxygenase activity decreased by 76% in ABT-treated rats and was not significantly altered in rats treated with HET0016. 20-HETE excretion rose from 470+/-21 to 570+/-41 ng/d when the rats were switched from the low to the high salt diet. 20-HETE excretion fell by 68% and 85% in rats that were chronically treated with ABT and HET0016. These results suggest that chronic blockade of the formation of 20-HETE promotes the development of salt-sensitive hypertension in rats.

Downregulation of renal CYP-derived eicosanoid synthesis in rats with diet-induced hypertension

The incidence of essential hypertension increases with obesity; however, the mechanisms that link obesity with hypertension are unclear. Renal cytochrome P450 (CYP)-derived eicosanoids--hydroxyeicosatetraenoic acids (HETEs), epoxyeicosatrienoic acids (EETs), and dihydroxyeicosatrienoic acids (DHETs)--have been shown to play an important role in the regulation of renal function, vascular tone, and blood pressure. The objective of this study was to examine CYP-derived eicosanoid synthesis in the different renal zones (cortex, medulla, and papilla) of rats fed a high fat diet (HF). Male Sprague-Dawley rats were fed a HF diet or regular rat chow for 10 weeks. After 10 weeks, HF rats showed significantly higher systolic blood pressure, body weight, and fat:body weight ratio. The renal omega-hydroxylase activity was decreased by 46% in cortex, 43% in medulla, and 46% in papilla of HF rats. The renal epoxygenase activity was decreased by 46% in cortex, 31% in medulla, and 56% in papilla of HF rats. Interestingly, the changes in the rate of 20-HETE and EET formation in different renal zones were consistent with the levels of expression of CYP4A and CYP2C23 proteins, respectively. Furthermore, there were no significant changes in the synthesis of these metabolites in the renal microvessels. These results demonstrate that HF diet causes the downregulation of CYP4A and CYP2C23 in renal tubules, and these proteins are responsible for renal 20-HETE and EET formation. The reduction in the synthesis of these eicosanoids may play an important role in the regulation of renal function and blood pressure in obesity-induced hypertension.

Differential regulation of natriuresis by 20-hydroxyeicosatetraenoic Acid in human salt-sensitive versus salt-resistant hypertension

BACKGROUND

Twenty-hydroxyeicosatetraenoic acid (20-HETE) is a cytochrome P450 metabolite of arachidonic acid that produces vasoconstriction and inhibition of renal tubular sodium transport. In Dahl rats, a 20-HETE deficiency plays a role in salt-sensitive (SS) hypertension. In humans, there are no data on regulation of 20-HETE by salt intake or on a role for this compound in SS hypertension.

METHODS AND RESULTS

Thirteen salt-resistant (SR) and 13 SS hypertensive subjects had urine 20-HETE excretion measured during salt-loading and depletion. In all patients, 20-HETE was 66.6% higher in the salt-replete (1.75+/-0.25 micro g/h) than in the salt-depleted state (1.05+/-0.16, P<0.003). There was no difference in 20-HETE excretion between SR and SS patients in either state of salt balance. In SR patients, sodium excretion during salt-loading correlated with 20-HETE (r=0.61, P<0.03) but not with blood pressure. In contrast, in SS patients, sodium excretion did not correlate with 20-HETE but did correlate with blood pressure (r=0.66, P<0.02). Finally, in the SS group only, there was a negative correlation between body mass index and 20-HETE excretion (r=-0.79, P<0.002) that was present during both salt-loading and depletion.

CONCLUSIONS

We demonstrate for the first time that 20-HETE excretion is regulated by salt intake in hypertension. We find a disrupted relationship between sodium excretion and 20-HETE in SS patients, which results in dependence of their salt excretion on blood pressure and may be related to the magnitude of their obesity. We conclude that salt-sensitivity of blood pressure in essential hypertension may result from impairment of a natriuretic mechanism dependent on 20-HETE.

Nitric oxide contributes to 20-HETE-induced relaxation of pulmonary arteries

In contrast to its constrictor effects on peripheral arteries, 20-hydroxyeicosatetraenoic acid (20-HETE) is an endothelial-dependent dilator of pulmonary arteries (PAs). The present study examined the hypothesis that the vasodilator effects of 20-HETE in PAs are due to an elevation of intracellular calcium concentration ([Ca(2+)](i)) and the release of nitric oxide (NO) from bovine PA endothelial cells (BPAECs). BPAECs express cytochrome P-450 4A (CYP4A) protein and produce 20-HETE. 20-HETE dilated PAs preconstricted with U-46619 or norepinephrine and treated with the cytochrome P-450 inhibitor 17-octadecynoic acid and the cyclooxygenase inhibitor indomethacin. The dilator effect of 20-HETE was blocked by the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) or by removal of endothelium. 20-HETE significantly increased [Ca(2+)](i) and NO production in BPAECs. 20-HETE-induced NO release was blunted by removal of extracellular calcium, as well as NO synthase inhibitors (L-NAME). These results suggest that 20-HETE dilates PAs at least in part by increasing [Ca(2+)](i) and NO release in BPAECs.

K depletion enhances the extracellular Ca2+-induced inhibition of the apical K channels in the mTAL of rat kidney

We have shown previously that raising extracellular Ca(2)+ inhibited the apical 70-pS K channel in the thick ascending limb (TAL; Wang, W.H., M. Lu, and S.C. Hebert. 1996. Am. J. Physiol. 270:C103-C111). We now used the patch-clamp technique to study the effect of increasing the extracellular Ca(2)+ on the 70-pS K channel in the mTAL from rats on a different K diet. Increasing the extracellular Ca(2)+ from 10 microM to 0.5, 1, and to 1.5 mM in the mTAL from rats on a K-deficient (KD) diet inhibited the channel activity by 30, 65, and 90%, respectively. In contrast, raising the extracellular Ca(2)+ to 1.5 mM had no significant effect on channel activity in the mTAL from animals on a high K (HK) diet and further increasing the extracellular Ca(2)+ to 2.5, 3.5, and 5.5 mM decreased the channel activity by 29, 55, and 90%, respectively. Inhibition of the cytochrome P450 monooxygenase completely abolished the effect of the extracellular Ca(2)+ on channel activity in the mTAL from rats on a different K diet. In contrast, blocking cyclooxygenase did not significantly alter the responsiveness of the 70-pS K channel to the extracellular Ca(2)+. Moreover, addition of sodium nitropruside, a nitric oxide (NO) donor, not only increased the channel activity, but also blunted the inhibitory effect of the extracellular Ca(2)+ on the 70-pS K channel and decreased 20-hydroxyeicosatetraenoic acid (20-HETE) concentration in the mTAL from rats on a KD diet. In contrast, inhibiting NOS with L-NAME enhanced the inhibitory effect of the extracellular Ca(2)+ on the channel activity and increased 20-HETE concentration in the mTAL from rats on a high K diet. Western blot has further shown that the expression of inducible NO synthase (iNOS) is significantly higher in the renal medulla from rats on an HK diet than that on a KD diet. Also, addition of S-nitroso-N-acetylpenicillamine abolished the inhibitory effect of arachidonic acid on channel activity in the mTAL, whereas it did not block the inhibitory effect of 20-HETE. We conclude that a low dietary K intake increases the sensitivity of the 70-pS K channel to the extracellular Ca(2)+, and that a decrease in NOS activity is involved in enhancing the inhibitory effect of the extracellular Ca(2)+ on channel activity in the mTAL during K depletion.

Induction of heme oxygenase 1 in radiation nephropathy: role of angiotensin II

Datta, P. K., Moulder, J. E., Fish, B. L., Cohen, E. P. and Lianos, E. A. Induction of Heme Oxygenase 1 in Radiation Nephropathy: Role of Angiotensin II. Radiat. Res. 155, 734-739 (2001). In a rat model of radiation-induced nephropathy, we investigated changes in expression of heme oxygenase 1 (Hmox1, also known as HO-1), an enzyme that catalyzes conversion of heme into biliverdin, carbon monoxide and iron. The study explored whether radiation induces Hmox1 expression in the irradiated kidney and whether angiotensin II (AII) mediates Hmox1 expression in glomeruli isolated from irradiated kidneys. To assess the effects of radiation on Hmox1 expression, rats received 20 Gy bilateral renal irradiation and were randomized to groups receiving an AII type 1 (AT(1)) receptor antagonist (L-158,809) or no treatment. Drug treatment began 9 days prior to bilateral renal irradiation and continued for the duration of the study. Estimation of Hmox1 levels in glomerular protein lysates assessed by Western blot analysis revealed a significant increase in Hmox1 protein at 50 and 65 days postirradiation. In animals treated with the AT(1) receptor antagonist, there was no induction of Hmox1, suggesting that AII may be a mediator of Hmox1 induction. To confirm that AII stimulates Hmox1 expression, animals were infused with 200, 400 or 800 ng/kg min(-1) of AII for 18-19 days, and Hmox1 protein levels in glomeruli were assessed. There was a significant induction of Hmox1 in glomeruli of animals infused with 800 ng/kg min(-1) of AII. These studies demonstrate that glomerular Hmox1 expression is elevated in the middle phase of radiation nephropathy and that AII can increase glomerular Hmox1 levels.

Effects of converting enzyme inhibitors on renal P-450 metabolism of arachidonic acid

The effects of blockade of the renin-angiotensin system on the renal metabolism of arachidonic acid (AA) were examined. Male Sprague-Dawley rats were treated with vehicle, captopril (25 mg x kg(-1) x day(-1)), enalapril (10 mg x kg(-1) x day(-1)), or candesartan (1 mg x kg(-1) x day(-1)) for 1 wk. The production of 20-hydroxyeicosatetraenoic acid (20-HETE) and epoxyeicosatrienoic acids (EETs) by renal cortical microsomes increased in rats treated with captopril by 59 and 24% and by 90 and 58% in rats treated with enalapril. Captopril and enalapril increased 20-HETE production in the outer medulla by 100 and 143%, respectively. In contrast, blockade of ANG II type 1 receptors with candesartan had no effect on the renal metabolism of AA. Captopril and enalapril increased cytochrome P-450 (CYP450) reductase protein levels in the renal cortex and outer medulla and the expression of CYP450 4A protein in the outer medulla. The effects of captopril on the renal metabolism of AA were prevented by the bradykinin-receptor antagonist, HOE-140, or the nitric oxide (NO) synthase inhibitor, N(G)-nitro-L-arginine methyl ester. These results suggest that angiotensin-converting enzyme inhibitors may increase the formation of 20-HETE and EETs secondary to increases in the intrarenal levels of kinins and NO.

Eicosanoid regulation of the renal vasculature

Even though it has been recognized that arachidonic acid metabolites, eicosanoids, play an important role in the control of renal blood flow and glomerular filtration, several key observations have been made in the past decade. One major finding was that two distinct cyclooxygenase (COX-1 and COX-2) enzymes exist in the kidney. A renewed interest in the contribution of cyclooxygenase metabolites in tubuloglomerular feedback responses has been sparked by the observation that COX-2 is constitutively expressed in the macula densa area. Arachidonic acid metabolites of the lipoxygenase pathway appear to be significant factors in renal hemodynamic changes that occur during disease states. In particular, 12(S)- hydroxyeicosatetraenoic acid may be important for the full expression of the renal hemodynamic actions in response to angiotensin II. Cytochrome P-450 metabolites have been demonstrated to possess vasoactive properties, act as paracrine modulators, and be a critical component in renal blood flow autoregulatory responses. Last, peroxidation of arachidonic acid metabolites to isoprostanes appears to be involved in renal oxidative stress responses. The recent developments of specific enzymatic inhibitors, stable analogs, and gene-disrupted mice and in antisense technology are enabling investigators to understand the complex interplay by which eicosanoids control renal blood flow.

Cytochrome P450-dependent renal arachidonic acid metabolism in desoxycorticosterone acetate-salt hypertensive mice

Cytochrome P450 (P450)-dependent arachidonic acid metabolites may act as mediators in the regulation of vascular tone and renal function. We studied arachidonic acid hydroxylase activities in renal microsomes from normotensive NMRI mice, desoxycorticosterone acetate (DOCA)-salt hypertensive mice, and DOCA-salt mice treated with either lovastatin or bezafibrate, both of which improve hemodynamics in this model. Control renal microsomes had arachidonic acid hydroxylase activities of 175+/-12 pmol. min(-1). mg(-1). The metabolites formed were 20- and 19-hydroxyarachidonic acid, representing approximately 80% and approximately 20% of the total hydroxylation. Treatment with DOCA-salt resulted in significantly decreased hydroxylase activities (to 84+/-4 pmol. min(-1). mg(-1)) of the total microsomal P450 content and a decrease in immunodetectable Cyp4a proteins. Lovastatin had no effect on these variables, whereas bezafibrate increased arachidonic acid hydroxylase activities to 163+/-12 pmol. min(-1). mg(-1). In situ hybridization with probes for Cyp4a-10, 12, and 14 revealed that Cyp4a-14 was the P450 isoform most strongly induced by bezafibrate. The expression was concentrated in the cortical medullary junction and was localized predominantly in the proximal tubules. In conclusion, these results suggest that the capacity to produce 20-hydroxyarachidonic acid is impaired in the kidneys of DOCA-salt hypertensive mice. Furthermore, bezafibrate may ameliorate hemodynamics in this model by restoring P450-dependent arachidonic acid hydroxylase activities. Lovastatin, on the other hand, exerts its effects via P450-independent mechanisms.

Effects of 20-HETE and 19(S)-HETE on rabbit proximal straight tubule volume transport

The kidney has the highest abundance of cytochrome P-450 of all extrahepatic organs. Within the kidney, the highest concentration of cytochrome P-450 is found in the proximal tubule. Whether 20- or 19(S)-hydroxyeicosatetraenoic acid (HETE), the major P-450 metabolites of arachidonic acid in the proximal tubule, affect transport in this segment has not been previously investigated. We examined the direct effects of 20- and 19(S)-HETE on volume absorption (J(v)) in the rabbit proximal straight tubule (PST). Production of 20-HETE by rabbit PST was demonstrated by incubating microdissected tubules with [(3)H]arachidonic acid and separating the lipid extract by HPLC. There was significant conversion of [(3)H]arachidonic acid to 20-HETE in control tubules that was inhibited by 10(-5) M N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS). Addition of exogenous 20-HETE had no effect on PST volume transport. However, inhibition of endogenous production of 20-HETE using DDMS stimulated transport. In the presence of DDMS, 20-HETE inhibited PST J(v). 19(S)-HETE in the bathing solution stimulated PST J(v) alone and in the presence of DDMS. Thus omega- and omega-1-hydroxylase products of arachidonic acid have direct effects on PST transport. Endogenous production of 20-HETE may play a role in tonic suppression of transport and may therefore be an endogenous regulator of transport in the proximal tubule.