Cytochrome P-450 arachidonic acid epoxygenase. Regulatory control of the renal epoxygenase by dietary salt loading

Soluble epoxide hydrolase inhibition lowers arterial blood pressure in angiotensin II hypertension

Epoxyeicosatrienoic acids (EETs) have antihypertensive properties and play a part in the maintenance of renal microvascular function. A novel approach to increase EET levels is to inhibit epoxide hydrolase enzymes that are responsible for conversion of biologically active EETs to dihydroxyeicosatrienoic acids (DHETs) that are void of effects on the preglomerular vasculature. We hypothesized that inhibition of soluble epoxide hydrolase (sEH) would lower blood pressure in angiotensin II (Ang II) hypertension. Rat renal cortical tissue was harvested and urine collected 2 weeks following implantation of an osmotic minipump containing Ang II (60 ng/min). Renal cortical sEH protein expression was significantly higher in Ang II hypertension compared with normotensive animals. Likewise, urinary 14,15-DHET levels were significantly increased in hypertensive compared with normotensive animals and averaged 8.1 +/- 1.3 and 2.7 +/- 1.1 ng/d; respectively. In additional experiments, the sEH inhibitor N-cyclohexyl-N-dodecyl urea (NCND; 3 mg/d) or vehicle (corn oil, 0.5 mL) was administered daily by intraperitoneal injection starting on day 10. Administration of NCND for 4 days lowered systolic blood pressure by 30 mm Hg in Ang II hypertensive animals, whereas the corn oil vehicle had no effect on blood pressure in normotensive or Ang II hypertensive animals. Measurement of blood pressure by indwelling arterial catheters in conscious animals with free movement in their cages confirmed that NCND had antihypertensive properties. Arterial blood pressure averaged 119 +/- 5 mm Hg in normotensive, 170 +/- 3 mm Hg in hypertensive and 149 +/- 10 mm Hg in NCND-treated, Ang II-infused animals. Administration of the potential metabolite of NCND, N-cyclohexylformamide to Ang II hypertensive rats did not lower the systolic blood pressure. These studies demonstrate that increased sEH expression in the Ang II hypertensive kidney leads to increased EET hydration. Moreover, sEH plays a role in the regulation of blood pressure, and inhibition of sEH during Ang II hypertension is antihypertensive.

P-450 metabolites of arachidonic acid in the control of cardiovascular function

Recent studies have indicated that arachidonic acid is primarily metabolized by cytochrome P-450 (CYP) enzymes in the brain, lung, kidney, and peripheral vasculature to 20-hydroxyeicosatetraenoic acid (20-HETE) and epoxyeicosatrienoic acids (EETs) and that these compounds play critical roles in the regulation of renal, pulmonary, and cardiac function and vascular tone. EETs are endothelium-derived vasodilators that hyperpolarize vascular smooth muscle (VSM) cells by activating K(+) channels. 20-HETE is a vasoconstrictor produced in VSM cells that reduces the open-state probability of Ca(2+)-activated K(+) channels. Inhibitors of the formation of 20-HETE block the myogenic response of renal, cerebral, and skeletal muscle arterioles in vitro and autoregulation of renal and cerebral blood flow in vivo. They also block tubuloglomerular feedback responses in vivo and the vasoconstrictor response to elevations in tissue PO(2) both in vivo and in vitro. The formation of 20-HETE in VSM is stimulated by angiotensin II and endothelin and is inhibited by nitric oxide (NO) and carbon monoxide (CO). Blockade of the formation of 20-HETE attenuates the vascular responses to angiotensin II, endothelin, norepinephrine, NO, and CO. In the kidney, EETs and 20-HETE are produced in the proximal tubule and the thick ascending loop of Henle. They regulate Na(+) transport in these nephron segments. 20-HETE also contributes to the mitogenic effects of a variety of growth factors in VSM, renal epithelial, and mesangial cells. The production of EETs and 20-HETE is altered in experimental and genetic models of hypertension, diabetes, uremia, toxemia of pregnancy, and hepatorenal syndrome. Given the importance of this pathway in the control of cardiovascular function, it is likely that CYP metabolites of arachidonic acid contribute to the changes in renal function and vascular tone associated with some of these conditions and that drugs that modify the formation and/or actions of EETs and 20-HETE may have therapeutic benefits.

EET homologs potently dilate coronary microvessels and activate BKCa channels

Epoxyeicosatrienoic acids (EETs) are released from endothelial cells and potently dilate small arteries by hyperpolarizing vascular myocytes. In the present study, we investigated the structural specificity of EETs in dilating canine and porcine coronary microvessels (50–140 μm ID) and activating large-conductance Ca2+-activated K+(BKCa) channels. The potencies and efficacies of EET regioisomers and enantiomers were compared with those of two EET homologs: epoxyeicosaquatraenoic acids (EEQs), which are made from eicosapentaenoic acid by the same cytochrome P-450 epoxygenase that generates EETs from arachidonic acid, and epoxydocosatetraenoic acids (EDTs), which are EETs that are two carbons longer. With EC50 values of 3–120 pM but without regio- or stereoselectivity, EETs potently dilated canine and porcine microvessels. Surprisingly, the EEQs and EDTs had comparable potencies and efficacies in dilating microvessels. Moreover, 50 nM 13,14-EDT activated the BKCa channels with the same efficacy as either 11,12-EET enantiomer at 50 nM. We conclude that coronary microvessels and BKCa channels possess low structural specificity for EETs and suggest that EEQs and EDTs may thereby also be endothelium-derived hyperpolarizing factors.

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.

Role of 20-HETE in mediating the effect of dietary K intake on the apical K channels in the mTAL

We have used the patch-clamp technique to study the effect of dietary K intake on the apical K channels in the medullary thick ascending limb (mTAL) of rat kidneys. The channel activity, defined by the number of channels in a patch and the open probability (NPo), of the 30- and 70-pS K channels, was 0.18 and 0.11, respectively, in the mTAL from rats on a K-deficient diet. In contrast, NPo of the 30- and 70-pS K channels increased to 0.60 and 0.80, respectively, in the tubules from animals on a high-K diet. The concentration of 20-hydroxyeicosatetraenoic acid (20-HETE) measured with gas chromatography-mass spectrometry was 0.8 pg/microg protein in the mTAL from rats on a high-K diet and increased significantly to 4.6 pg/microg protein in the tubules from rats on a K-deficient diet. Addition of N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS) or 17-octadecynoic acid (17-ODYA), agents that inhibit the formation of 20-HETE, had no significant effect on the activity of the 30-pS K channels. However, DDMS/17-ODYA significantly increased the activity of the apical 70-pS K channel from 0.11 to 0.91 in the mTAL from rats on a K-deficient diet. In contrast, inhibition of the cytochrome P-450 metabolism of arachidonic acid increased NPo from 0.64 to 0.81 in the tubules from animals on a high-K diet. Furthermore, the sensitivity of the 70-pS K channel to 20-HETE was the same between rats on a high-K diet and on a K-deficient diet. Finally, the pretreatment of the tubules with DDMS increased NPo of the 70-pS K channels in the mTAL from rats on a K-deficient diet to 0.76. We conclude that an increase in 20-HETE production is involved in reducing the activity of the apical 70-pS K channels in the mTAL from rats on a K-deficient diet.

Targeted disruption of soluble epoxide hydrolase reveals a role in blood pressure regulation

Renal microsomal cytochrome P-450 monooxygenase-dependent metabolism of arachidonic acid generates a series of regioisomeric epoxyeicosatrienoic acids that can be further metabolized by soluble epoxide hydrolase to the corresponding dihydroxyeicosatrienoic acids. Evidence exists that these metabolites affect renal function and, in particular, blood pressure regulation. To examine this possibility, blood pressure and renal arachidonic acid metabolism were examined in mice with a targeted disruption of the soluble epoxide hydrolase gene. Systolic blood pressure of male soluble epoxide hydrolase-null mice was lower compared with wild-type mice in both the absence and presence of dietary salt loading. Both female soluble epoxide hydrolase-null and wild-type female mice also had significantly lower systolic blood pressure than male wild-type mice. Renal formation of epoxyeicosatrienoic and dihydroxyeicosatrienoic acids was markedly lower for soluble epoxide hydrolase-null versus wild-type mice of both sexes. Although disruption of soluble epoxide hydrolase in female mice had minimal effects on blood pressure, deletion of this gene feminized male mice by lowering systolic blood pressure and altering arachidonic acid metabolism. These data provide the first direct evidence for a role for soluble epoxide hydrolase in blood pressure regulation and identify this enzyme as a novel and attractive target for therapeutic intervention in hypertension.

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.

Soluble epoxide hydrolase regulates hydrolysis of vasoactive epoxyeicosatrienoic acids

The cytochrome P450-derived epoxyeicosatrienoic acids (EETs) have potent effects on renal vascular reactivity and tubular sodium and water transport; however, the role of these eicosanoids in the pathogenesis of hypertension is controversial. The current study examined the hydrolysis of the EETs to the corresponding dihydroxyeicosatrienoic acids (DHETs) as a mechanism for regulation of EET activity and blood pressure. EET hydrolysis was increased 5- to 54-fold in renal cortical S9 fractions from the spontaneously hypertensive rat (SHR) relative to the normotensive Wistar-Kyoto (WKY) rat. This increase was most significant for the 14,15-EET regioisomer, and there was a clear preference for hydrolysis of 14, 15-EET over the 8,9- and 11,12-EETs. Increased EET hydrolysis was consistent with increased expression of soluble epoxide hydrolase (sEH) in the SHR renal microsomes and cytosol relative to the WKY samples. The urinary excretion of 14,15-DHET was 2.6-fold higher in the SHR than in the WKY rat, confirming increased EET hydrolysis in the SHR in vivo. Blood pressure was decreased 22+/-4 mm Hg (P:<0.01) 6 hours after treatment of SHRs with the selective sEH inhibitor N:, N:'-dicyclohexylurea; this treatment had no effect on blood pressure in the WKY rat. These studies identify sEH as a novel therapeutic target for control of blood pressure. The identification of a potent and selective inhibitor of EET hydrolysis will be invaluable in separating the vascular effects of the EET and DHET eicosanoids.

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.

Fluorescent HPLC assay for 20-HETE and other P-450 metabolites of arachidonic acid

This study describes a fluorescent HPLC assay for measuring 20-hydroxyeicosatetraenoic acid (20-HETE) and other cytochrome P-450 metabolites of arachidonic acid in urine, tissue, and interstitial fluid. An internal standard, 20-hydroxyeicosa-6(Z),15(Z)-dienoic acid, was added to samples, and the lipids were extracted and labeled with 2-(2,3-naphthalimino)ethyl trifluoromethanesulfonate. P-450 metabolites were separated on a C18 reverse-phase HPLC column. Coelution and gas chromatography-mass spectrometry studies confirmed the identity of the 20-HETE peak. The 20-HETE peak can be separated from those for dihydroxyeicosatrienoic acids, other HETEs, and epoxyeicosatrienoic acids. Known amounts of 20-HETE were used to generate a standard curve (range 1-10 ng, r(2) = 0.98). Recovery of 20-HETE from urine averaged 95%, and the intra-assay variation was <5%. Levels of 20-HETE were measured in 100 microliter of urine and renal interstitial fluid or 0.1 mg of renal tissue. The assay was evaluated by studying the effects of 1-aminobenzotriazole (ABT) on the excretion of 20-HETE in rats. ABT reduced excretion of 20-HETE by >65% and inhibited the formation of 20-HETE by renal microsomes. The availability of this assay should facilitate work in this field.

Angiotensin II modulates ion transport in rat proximal tubules through CYP metabolites

To assess the effect of angiotensin II on ion transport in rat isolated proximal tubules and establish the arachidonic acid cytochrome P450 metabolites' role mediating angiotensin II effect and to analyze whether corticosteroids play a role modulating this effect, we studied the effect of low (10 and 100 pM) and high (0.1-1 microM) angiotensin II concentrations on proximal tubule ion transport, measured as (86)Rb uptake. Low angiotensin II produced a stimulation on the (86)Rb uptake (195.79 +/- 35, 377.9 +/- 81, and 300 +/- 49 pg (86)Rb/microg protein/2 min, for control and 10 and 100 pM angiotensin II, respectively). High angiotensin II concentration inhibited ion transport (0.1 microM, 57.9 +/- 5 and 1 microM, 47.3 +/- 4 pg (86)Rb/microg protein/2 min), this effect was prevented by 17-ODYA and by losartan, while indomethacin had no effect. Dexamethasone treatment increased angiotensin II-induced (86)Rb uptake inhibition and arachidonic acid metabolism (19-, 20-HETE and 12-HETE), while adrenalectomy partly prevented angiotensin II-induced inhibition and decreased cytochrome P450-dependent arachidonic acid metabolism. In conclusion, high doses of angiotensin II produce inhibition of ion transport in rat isolated proximal tubules; this effect is mediated by AT(1) receptors, involves cytochrome P450-dependent arachidonic acid metabolites, and is upregulated by corticosteroids.

Biochemical characterization of rat P450 2C11 fused to rat or bacterial NADPH-P450 reductase domains

cDNAs coding for rat P450 2C11 fused to either a bacterial (the NADPH-cytochrome P450 BM3 reductase domain of P450 BM3) or a truncated form of rat NADPH-P450 reductases were expressed in Escherichia coli and characterized enzymatically. Measurements of NADPH cytochrome c reductase activity showed fusion-dependent increases in the rates of cytochrome c reduction by the bacterial or the mammalian flavoprotein (21 and 48%, respectively, of the rates observed with nonfused enzymes). Neither the bacterial flavoprotein nor the truncated rat reductase supported arachidonic acid metabolism by P450 2C11. In contrast, fusion of P450 2C11 to either reductase yielded proteins that metabolized arachidonic acid to products similar to those obtained with reconstituted systems containing P450 2C11 and native rat P450 reductase. Addition of a 10-fold molar excess of rat P450 reductase markedly increased the rates of metabolism by both fused and nonfused P450s 2C11. These increases occurred with preservation of the regioselectivity of arachidonic acid metabolism. The fusion-independent reduction of P450 2C11 by bacterial P450 BM3 reductase was shown by measurements of NADPH-dependent H(2)O(2) formation [73 +/- 10 and 10 +/- 1 nmol of H(2)O(2) formed min(-)(1) (nmol of P450)(-)(1) for the reconstituted and fused protein systems, respectively]. These studies demonstrate that (a) a self-sufficient, catalytically active arachidonate epoxygenase can be constructed by fusing P450 2C11 to mammalian or bacterial P450 reductases and (b) the P450 BM3 reductase interacts efficiently with mammalian P450 2C11 and catalyzes the reduction of the heme iron. However, fusion is required for metabolism and product formation.

Eicosanoids and the stomach

Eicosanoids are arachidonic acid derivatives that include prostaglandins, thromboxanes, and leukotrienes. During the last three decades, it has become evident that these bioactive lipids play a pivotal role in gastric physiology. The goal of the present review is to describe their involvement in the normal regulation of gastric secretion and gastric motility, as well as in gastric mucosal defense. Their role in gastric mucosal mitogenesis, apoptosis, inflammation, and immune modulatory responses is also discussed.

Eicosanoids and the small intestine

Prostaglandins play an important role in modulation of various physiologic processes in the small intestine. In this review, the involvement of prostaglandins in various small-intestinal functions including small-intestinal secretion, mucosal protection, epithelial and endothelial barrier function, and motility are discussed.

Soluble beta-amyloid peptides mediate vasoactivity via activation of a pro-inflammatory pathway

Freshly solubilized beta-amyloid (Abeta) peptides display vasoactive properties, increasing both the magnitude and the duration of endothelin-1-induced vasoconstriction. We show that Abeta vasoactivity is mediated by the stimulation of a pro-inflammatory pathway involving activation of secretory phospholipase A(2) (PLA(2)), mitogen activated protein kinase (MAPK) kinase (MEK1/2), p38 MAPK, cytosolic PLA(2), and the release of arachidonic acid. Ultimately, arachidonic acid is metabolized into proinflammatory eicosanoids via the 5-lipoxygenase and cyclooxygenase-2 (COX-2) enzymes, both of which we show to be required for A beta vasoactivity. Accordingly, p38 MAPK activity is higher in the brains of transgenic mice that overproduce A beta, and COX-2 immunoreactivity is increased in the cerebrovasculature of these transgenic animals. Taken together, our data show that freshly solubilized A beta peptides can trigger a pro-inflammatory reaction in the vasculature that can be blocked by inhibiting specific target molecules, providing the basis for novel therapeutic intervention.

Formation of 20-hydroxyeicosatetraenoic acid, a vasoactive and natriuretic eicosanoid, in human kidney. Role of Cyp4F2 and Cyp4A11

20-hydroxyeicosatetraenoic acid (20-HETE), an omega-hydroxylated arachidonic acid (AA) metabolite, elicits specific effects on kidney vascular and tubular function that, in turn, influence blood pressure control. The human kidney's capacity to convert AA to 20-HETE is unclear, however, as is the underlying P450 catalyst. Microsomes from human kidney cortex were found to convert AA to a single major product, namely 20-HETE, but failed to catalyze AA epoxygenation and midchain hydroxylation. Despite the monophasic nature of renal AA omega-hydroxylation kinetics, immunochemical studies revealed participation of two P450s, CYP4F2 and CYP4A11, since antibodies to these enzymes inhibited 20-HETE formation by 65. 9 +/- 17 and 32.5 +/- 14%, respectively. Western blotting confirmed abundant expression of these CYP4 proteins in human kidney and revealed that other AA-oxidizing P450s, including CYP2C8, CYP2C9, and CYP2E1, were not expressed. Immunocytochemistry showed CYP4F2 and CYP4A11 expression in only the S2 and S3 segments of proximal tubules in cortex and outer medulla. Our results demonstrate that CYP4F2 and CYP4A11 underlie conversion of AA to 20-HETE, a natriuretic and vasoactive eicosanoid, in human kidney. Considering their proximal tubular localization, these P450 enzymes may partake in pivotal renal functions, including the regulation of salt and water balance, and arterial blood pressure itself.

Age-related changes in renal cyclic nucleotides and eicosanoids in response to sodium intake

The signaling molecules cGMP, cAMP, prostaglandin E(2) (PGE(2)), and prostaglandin F(2alpha) (PGF(2alpha)) play important roles in mediating the response of the kidney to changes in dietary sodium intake. We used a renal microdialysis technique in conscious rats to address the hypothesis that the renal ability to produce these mediators in response to dietary sodium intake is altered during maturation. Young (4-week-old) or adult (6-month-old) rats were studied after the consumption for 5 days of diets containing low (0. 04% NaCl), normal (0.28% NaCl), or high (4.0% NaCl) levels of sodium. Plasma renin activity was significantly increased by low-sodium diet and significantly decreased by high-sodium diet, with no significant difference between the responses of the 2 age groups. Renal interstitial fluid (RIF) levels of cGMP, cAMP, PGE(2), and PGF(2alpha) on normal-sodium diet were similar in the 2 age groups. Low-sodium diet caused a significant increase in RIF levels of all 4 mediators, with no significant differences between the responses of the 2 age groups. High-sodium diet also caused a significant increase in RIF levels of all 4 mediators. However, RIF production of cGMP, cAMP, and PGE(2) was significantly greater, and RIF PGF(2alpha) production was significantly lower, in young rats compared with adult rats. These data demonstrate that the kidneys of young and adult rats respond to dietary sodium restriction in a similar manner but that there are age-related changes in the renal response to sodium loading.

Exaggerated volume expansion natriuresis in rats preloaded with hypertonic saline: a paradoxical enhancement by inhibition of prostaglandin synthesis

In preliminary experiments rats preinfused with hypertonic saline showed exaggerated natriuresis after an additional small volume expansion (SVE). This was systematically studied in anaesthetized Wistar rats prepared for clearance studies of the left kidney and measurements of medullary blood flow (MBF, laser-Doppler technique) and tissue electrical admittance (Y ), an index of interstitial ion concentration. The rats were preinfused i.v. with 3 mL of 5% NaCl during 90 min. A subsequent injection of isotonic saline, 0.5% of body weight, increased sodium excretion (UNaV ) from 2.1 +/- 0.5 to 4.5 +/- 1.1 micromol min-1 and urine flow (V ) from 12.0 +/- 2.3 to 24.3 +/- 5.6 microL min-1 (P < 0.02). The same volume of whole blood increased UNaV from 5.0 +/- 1.4 to 8.7 +/- 1.7 micromol min-1 and V from 22.3 +/- 5.1 to 37.4 +/- 5.9 microL min-1 (P < 0.01). The glomerular filtration rate, MBF and Y did not change. In rats preinfused with 0.9% saline no natriuresis was observed after SVE. To examine if prostaglandins (PG) were involved in SVE natriuresis, indomethacin (Indo), 5 mg kg-1 or sodium meclophenamate (Meclo), 7.5 mg kg-1, were added to the injected 0.9% saline. Paradoxically, both PG synthesis inhibitors enhanced natriuresis to SVE. After Indo UNaV increased from 2.0 +/- 0.6 to 7.6 +/- 1.3 micromol min-1, significantly more than after SVE alone (P < 0.001). At higher baseline UNaV, the increase with Meclo from 4.5 +/- 1.2 to 13.5 +/- 1.8 micromol min-1 was significantly higher than after whole blood infusion (P < 0.001). MBF decreased and Y increased after both inhibitors. Further studies are required to explain the enhancement of natriuresis after blockade of PG synthesis.

The kidney cytochrome P-450 2C23 arachidonic acid epoxygenase is upregulated during dietary salt loading

Excess dietary salt intake induces the activity of the kidney arachidonate epoxygenase and markedly increases the urinary excretion of its metabolites. The epoxyeicosatrienoic acids, products of the kidney P-450 arachidonate epoxygenase, inhibit distal nephron Na(+) reabsorption. Nucleic acid hybridization studies demonstrated the expression of P-450s 2C23, 2C24, and 2C11 as the predominant kidney 2C isoforms and the lack of significant dietary salt-dependent transcriptional regulation of these proteins. Recombinant P-450s 2C11, 2C23, and 2C24 catalyze arachidonate metabolism to mixtures of epoxy- and monohydroxylated acids. Whereas the arachidonate 11,12-olefin was the preferred target for epoxidation by P-450 2C23 (57% of total products), P-450s 2C11 and 2C24 epoxidized the 11,12-olefins and 14,15-olefins with nearly equal efficiency. Stereochemical comparisons demonstrated that the regiochemical and enantiofacial selectivity of P-450 2C23 matched that of the kidney microsomal epoxygenase and that excess dietary salt does not alter the regiochemical or stereochemical selectivity of the kidney arachidonate epoxygenase. Inhibition and immunoelectrophoresis experiments using antibodies raised against recombinant P-450s 2C11 and 2C23 demonstrated that P-450 2C23 is the major 2C arachidonic acid epoxygenase in the rat kidney and the renal P-450 isoform regulated by excess dietary salt intake.

20-HETE and the kidney: resolution of old problems and new beginnings

The protean properties of 20-hydroxyeicosatetraenoic acid (HETE), vasoactivity, mitogenicity, and modulation of transport in key nephron segments, serve as the basis for the essential roles of 20-HETE in the regulation of the renal circulation and electrolyte excretion and as a second messenger for endothelin-1 and mediator of selective renal effects of ANG II. Renal autoregulation and tubular glomerular feedback are mediated by 20-HETE through constriction of preglomerular arterioles, responses that are maintained by 20-HETE inhibition of calcium-activated potassium channels. 20-HETE modulates ion transport in the proximal tubules and the thick ascending limb by affecting the activities of Na+-K+-ATPase and the Na+-K+-2Cl- cotransporter, respectively. The range and diversity of activity of 20-HETE derives in large measure from COX-dependent transformation of 20-HETE to products affecting vasomotion and salt and water excretion. Nitric oxide (NO) exerts a negative modulatory effect on 20-HETE formation; inhibition of NO synthesis produces marked perturbation of renal function resulting from increased 20-HETE production. 20-HETE is an essential component of interactions involving several hormonal systems that have central roles in blood pressure homeostasis, including angiotensins, endothelins, NO, and cytokines. 20-HETE is the preeminent renal eicosanoid, overshadowing PGE2 and PGI2. This review is intended to provide evidence for the physiological roles for cytochrome P-450-derived eicosanoids, particularly 20-HETE, and seeks to extend this knowledge to a conceptual framework for overall cardiovascular function.

Molecular cloning, enzymatic characterization, developmental expression, and cellular localization of a mouse cytochrome P450 highly expressed in kidney

A cDNA encoding a new cytochrome P450 was isolated from a mouse liver library. Sequence analysis reveals that this 1,886-base pair cDNA encodes a 501-amino acid polypeptide that is 69-74% identical to CYP2J subfamily P450s and is designated CYP2J5. Recombinant CYP2J5 was co-expressed with NADPH-cytochrome P450 oxidoreductase in Sf9 cells using a baculovirus system. Microsomal fractions of CYP2J5/NADPH-cytochrome P450 oxidoreductase-transfected cells metabolize arachidonic acid to 14,15-, 11,12-, and 8, 9-epoxyeicosatrienoic acids and 11- and 15-hydroxyeicosatetraenoic acids (catalytic turnover, 4.5 nmol of product/nmol of cytochrome P450/min at 37 degrees C); thus CYP2J5 is enzymologically distinct. Northern analysis reveals that CYP2J5 transcripts are most abundant in mouse kidney and present at lower levels in liver. Immunoblotting using a polyclonal antibody against a CYP2J5-specific peptide detects a protein with the same electrophoretic mobility as recombinant CYP2J5 most abundantly in mouse kidney microsomes. CYP2J5 is regulated during development in a tissue-specific fashion. In the kidney, CYP2J5 is present before birth and reaches maximal levels at 2-4 weeks of age. In the liver, CYP2J5 is absent prenatally and during the early postnatal period, first appears at 1 week, and then remains relatively constant. Immunohistochemical staining of kidney sections with anti-human CYP2J2 IgG reveals that CYP2J protein(s) are present primarily in the proximal tubules and collecting ducts, sites where the epoxyeicosatrienoic acids are known to modulate fluid/electrolyte transport and mediate hormonal action. In situ hybridization confirms abundant CYP2J5 mRNA within tubules of the renal cortex and outer medulla. Epoxyeicosatrienoic acids are endogenous constituents of mouse kidney thus providing direct evidence for the in vivo metabolism of arachidonic acid by the mouse renal epoxygenase(s). Based on these data, we conclude that CYP2J5 is an enzymologically distinct, developmentally regulated, protein that is localized to specific nephron segments and contributes to the oxidation of endogenous renal arachidonic acid pools. In light of the well documented effects of epoxyeicosatrienoic acids in modulating renal tubular transport processes, we postulate that CYP2J5 products play important functional roles in the kidney.

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

We recently reported that an enzyme of the cytochrome P-450 4A family is expressed in the glomerulus, but there is no evidence that 20-hydroxyeicosatetraenoic acid (20-HETE) can be produced by this tissue. The purpose of present study was to determine whether glomeruli isolated from the kidney of rats can produce 20-HETE and whether the production of this metabolite is regulated by nitric oxide (NO) and dietary salt intake. Isolated glomeruli produced 20-HETE, dihydroxyeicosatrienoic acids, and 12-hydroxyeicosatetraenoic acid (4.13 +/- 0.38, 4.20 +/- 0.38, and 2. 10 +/- 0.20 pmol. min-1. mg protein-1, respectively) when incubated with arachidonic acid (10 microM). The formation of 20-HETE was dependent on the availability of NADPH and the PO2 of the incubation medium. The formation of 20-HETE was inhibited by NO donors in a concentration-dependent manner. The production of 20-HETE was greater in glomeruli isolated from the kidneys of rats fed a low-salt diet than in kidneys of rats fed a high-salt diet (5.67 +/- 0.32 vs. 2.83 +/- 0.32 pmol. min-1. mg protein-1). Immunoblot experiments indicated that the expression of P-450 4A protein in glomeruli from the kidneys of rats fed a low-salt diet was sixfold higher than in kidneys of rats fed a high-salt diet. These results indicate that arachidonic acid is primarily metabolized to 20-HETE and dihydroxyeicosatrienoic acids in glomeruli and that glomerular P-450 activity is modulated by NO and dietary salt intake.

Endothelin-1 and CYP450 arachidonate metabolites interact to promote tissue injury in DOCA-salt hypertension

Inhibition of cytochrome P-450 (CYP450) enzymes with cobalt chloride (CoCl2) prevented hypertension, organ hypertrophy, and renal injury induced by DOCA and salt (1% NaCl) in uninephrectomized (UNx) rats. Systolic blood pressure (SBP) rose to 193 +/- 6 mmHg by day 21 from control levels of 150 +/- 7 mmHg in response to DOCA-salt treatment, a rise that was prevented by CoCl2 (24 mg. kg-1. 24 h-1). The effects of DOCA-salt treatment, which increased protein excretion to 88.3 +/- 6.9 mg/24 h on day 21 from 9.0 +/- 1.1 mg/24 h on day 3, were prevented by CoCl2. CoCl2 also attenuated the renal and left ventricular hypertrophy and the increase in media-to-lumen ratio in hypertensive rats. DOCA-salt treatment increased excretion of endothelin (ET)-1 from 81 +/- 17 to 277 +/- 104 pg. 100 g body wt-1. 24 h-1 associated with a fourfold increase in 20-hydroxyeicosatetraenoic acid (20-HETE) excretion from 3.0 +/- 1.1 to 12.2 +/- 1.9 ng. 100 g body wt-1. 24 h-1 (days 3 vs. 21). CoCl2 blunted these increases by 58 and 72%, respectively. In aortic rings pulsed with [3H]thymidine, ET-1 increased its incorporation. Dibromododec-11-enoic acid, an inhibitor of 20-HETE synthesis, attenuated ET-1-induced increases in [3H]thymidine incorporation. We distinguished effects of CoCl2 acting via CO generation vs. suppression of CYP450-arachidonic acid metabolism by treating UNx-salt-DOCA rats with 1-aminobenzotriazole (ABT), which suppresses CYP450 enzyme activity, and compared these results to those produced by CoCl2. ABT reduced hypertension, as did CoCl2. Unlike CoCl2, ABT did not prevent organ hypertrophy and proteinuria, suggesting that these effects were partially related to CO formation. Blockade of the ETA receptor with BMS-182874 reduced SBP, organ hypertrophy, and proteinuria, indicating the importance of ET-initiated abnormalities to the progression of lesions in UNx-salt-DOCA.

P450 subfamily CYP2J and their role in the bioactivation of arachidonic acid in extrahepatic tissues

Historically, there has been intense interest in P450 metabolic oxidation, peroxidation, and reduction of xenobiotics. More recently, there has been a growing appreciation for the role of P450s in the oxidation of lipophilic endobiotics, such as bile acids, fat-soluble vitamins, and eicosanoids. This review details the emerging CYP2J subfamily of P450s and their role as catalysts of arachidonic acid metabolism.

The effect of selective cyclooxygenase inhibitors on intestinal epithelial cell mitogenesis

INTRODUCTION

Previous research has demonstrated that nonsteroidal anti-inflammatory agents alter the incidence of colorectal cancer. It has been postulated that the response may be due to the effect of these agents on the activities of the cyclooxygenase (COX) enzymes. The COX enzymes catalyze the conversion of arachidonic acid to biologically active prostanoids. Two forms of COX have been identified. COX-1 is a constitutive enzyme, generally involved in cell functions, while COX-2 is commonly an enzyme which is inducible in response to various stimuli, including mitogens. Recently, specific inhibitors of COX-1 and COX-2 enzymes have been developed.

PURPOSE

The present study was undertaken to determine the effects of specific COX-1 and COX-2 inhibitors on the proliferation and the induction of apoptosis of intestinal epithelial cells.

METHODS

A continuously proliferating rat small intestinal cell line (IEC-18) and a mouse colon cancer cell line (WB-2054) were utilized for these experiments. The cells were placed in microwells with serum-free or serum-supplemented media. The effects of serum on proliferation were then evaluated in the presence of the COX-1 inhibitor, valerylsalicyclic acid (VSA), the COX-2 inhibitor, SC-58125, or indomethacin. The presence of COX-1 and COX-2 protein was evaluated by Western blotting. Proliferation of intestinal cells was quantitated by incorporation of [3H]thymidine into DNA and cell counting, and apoptosis was determined by evaluating cell attachment. COX activity was evaluated by prostaglandin E2 production measured by enzyme-linked immunoabsorbent assay (ELISA).

RESULTS

Western blotting of IEC-18 and WB-2054 cell protein demonstrated COX-1 enzyme in cells incubated in serum-free media with increased COX-1 expression produced by incubation in media supplemented with 10% serum. COX-2 enzyme was not demonstrated in serum-free media; however, it was present in cells maintained in 10% serum-supplemented media. Spontaneous DNA synthesis was present in both cell lines and serum increased proliferation. In both cell lines [3H]thymidine incorporation stimulated by serum was inhibited by the COX-2 inhibitor SC-58125, but not by the COX-1 inhibitor VSA. Both indomethacin and SC-58125 produced a dose-dependent increase in apoptotic ratios in both cell lines. PGE2 formation, stimulated by serum, was inhibited by SC-58125, VSA, and indomethacin.

CONCLUSION

A differential effect on intestinal cell mitogenesis was seen with different COX inhibitors. The COX-2 inhibitor, but not the COX-1 inhibitor, significantly inhibited [3H]thymidine incorporation in both cell types, suggesting COX-2 inhibitors may be specific inhibitors of normal epithelial cell proliferation and growth of malignant cells. SC-58125, a selective inhibitor of COX-2, has a potent apoptosis inducing effect. The inhibition of PGE2 production did not correlate with the inhibition of proliferation, suggesting the two processes are unrelated.

Afferent arteriolar vasodilation to the sulfonimide analog of 11, 12-epoxyeicosatrienoic acid involves protein kinase A

The current study determined the contribution of protein kinase-A (PKA) and protein kinase-G (PKG) to the vasodilation elicited by the N-methylsulfonimide analog of 11,12-epoxyeicosatrienoic acid (11, 12-EET). Experiments were performed, in vitro, using the juxtamedullary nephron preparation combined with videomicroscopy. The response of afferent arterioles to the sulfonimide analog of 11, 12-EET, was determined before and after inhibition of PKA, PKG, or guanylyl cyclase. Afferent arterioles, preconstricted with 0.5 micromol/L norepinephrine, averaged 18+/-1 microm (n=25) at a renal perfusion pressure of 100 mm Hg. Superfusion with 0.01 to 100 nmol/L of the 11,12-EET analog caused a graded increase in diameter of the afferent arteriole. Vessel diameter increased by 11+/-1% and 15+/-1%, respectively, in response to 10 and 100 nmol/L of the 11,12-EET analog. The afferent arteriolar response to 10 and 100 nmol/L of the 11,12-EET analog was significantly attenuated during inhibition of PKA with 10 micromol/L H-89 (n=7) or 5 micromol/L myristolated PKI (n=6), such that afferent arteriolar diameter increased by only 5+/-2% and 2+/-1%, respectively, in response to 100 nmol/L of the 11, 12-EET analog. In contrast, the afferent arteriolar vasodilatory response to the 11,12-EET analog was unaffected by PKG or guanylyl cyclase inhibition. In the presence of 200 micromol/L histone H2B (n=5) or 10 micromol/L ODQ (n=7), the afferent arteriolar diameter increased by 16+/-3% and 12+/-2%, respectively, in response to 100 nmol/L of the 11,12-EET analog. These results demonstrate that activation of PKA is an important mechanism responsible for the afferent arteriolar vasodilation elicited by the sulfonimide analog of 11,12-EET.

Molecular, enzymatic, and regulatory characterization of rat kidney cytochromes P450 4A2 and 4A3

The cDNAs encoding cytochromes CYP 4A2 and 4A3 were cloned by RT-PCR amplification of male rat kidney and liver RNAs, respectively. Sequence analysis demonstrated that these cDNAs were nearly identical to the published sequences for CYPs 4A2 and 4A3. CYP 4A2 and 4A3 share extensive sequence homology that extends into their 3'- and 5'-untranslated segments ( approximately 97% overall nucleotide identity). Analysis of cDNA and genomic DNA sequences shows that a sequence of 123 bp, recognized as an intron during the processing of CYP 4A2 transcripts, is conserved in the 4A3 mRNAs and that these otherwise highly homologous genes show different exon-intron distributions. The CYP 4A2 and 4A3 cDNAs were expressed in a baculovirus-insect cell expression system. Purified recombinant CYP 4A2 oxidized arachidonic acid to a mixture of 19- and 20-hydroxyeicosatetraenoic acids (20 and 80% of the total products, respectively). Reaction rates were maximal when CYP 4A2 was reconstituted in the presence of an equimolar concentration of cytochrome b5 and a 10-fold molar excess of NADPH-cytochrome P450 reductase. Studies using microsomal fractions isolated from noninfected insect cells and from cells infected with CYP 4A3 recombinant baculoviruses showed (a) the presence of an endogenous lauric acid omega-hydroxylase and arachidonic acid epoxygenase in the noninfected cells, (b) the CYP 4A3-dependent oxidation of lauric acid to 11- and 12-hydroxylaurate (24 and 76% of the total products, respectively), and (c) the lack of arachidonic acid metabolism by microsomal recombinant CYP 4A3. Nucleic acid hybridization and immunoelectrophoresis studies demonstrated that (a) CYP 4A2 transcripts are abundantly expressed in the female kidney and that CYP 4A3 is expressed in female but not in male liver, (b) anti-CYP 4A2 immunoreactive material was detected only in the male kidney, (c) male and female livers or kidneys support only low levels of CYP 4A3 translation, and (d) excess dietary salt does not alter the kidney levels of mRNA transcripts encoding CYP 4A1, 4A2, or 4A3 or change the levels of microsomal anti-4A1 or -4A2 immunoreactive proteins. Finally, no significant differences were observed between Dahl salt resistant or Dahl salt sensitive rats in the levels and/or salt regulation of mRNA transcripts enecoding CYP 4A1, 4A2, or 4A3 or the in levels of the corresponding proteins.

Epoxyeicosatrienoic acids relax airway smooth muscles and directly activate reconstituted KCa channels

Epoxyeicosatrienoic acids (EETs) relax various smooth muscles by increasing outward K+ movement, but the molecular mode of action of EET regioisomers remains to be clarified. The effects of EETs were investigated on bovine airway smooth muscle tone and on reconstituted Ca2+-activated K+ (KCa) channels. 5,6-EET and 11, 12-EET induced dose-dependent relaxations of precontracted bronchial spirals. These effects were partly abolished by 10 nM iberiotoxin. Bilayer experiments have shown that 0.1-10 microM 11,12-EET produced up to fourfold increases in the open probability of KCa channels from the cis (extracellular) side by enhancing the mean open time constant and reducing the long closed time constant, without affecting the unitary conductance. EET-induced activations were blocked by 10 nM iberiotoxin. Addition of vehicles or other lipids as well as of GTP and guanosine 5'-O-(3-thiotriphosphate) in the absence of EET had no effect on channel activity. Thus EETs directly activate KCa channels from airway smooth muscle through an interaction with the extracellular face of the channel. We propose that EETs could represent candidate molecules as epithelium-derived hyperpolarizing factors.

Nutritional status modulates rat liver cytochrome P450 arachidonic acid metabolism

Alterations in nutritional status affect hepatic cytochrome P450 levels. Since cytochromes P450 participate in the metabolism of arachidonic acid, we hypothesized that changes in liver P450 arachidonic acid metabolism occur during fasting and refeeding. Male Fisher 344 rats were either fed, fasted 48 hr (F48), fasted 48 hr and then refed 6 hr (F48/R6), or fasted 48 hr and then refed 24 hr (F48/R24). F48 rats had reduced body weight, increased plasma beta-hydroxybutyrate, and reduced plasma insulin compared with the other groups. Although there was no significant change in total liver P450 content, there was a significant 20%, 48%, and 24% reduction in total hepatic microsomal arachidonic acid metabolism in F48, F48/R6, and F48/R24 rats, respectively, compared with fed rats. Epoxygenase activity decreased by 28%, 51%, and 26% in F48, F48/R6, and F48/R24 rats, respectively. In contrast, omega-1 hydroxylase activity increased by 126% in F48 rats compared with fed rats. Immunoblotting revealed that levels of CYP2C11 protein were markedly reduced, whereas levels of CYP2E1 protein were markedly increased in the F48 and F48/R6 groups. In contrast, levels of CYP1A1, CYP1A2, CYP2B1, CYP2J3, CYP4A1, and CYP4A3 were unchanged with fasting/refeeding. Northern blots revealed that levels of CYP2C11 mRNAs were decreased, whereas CYP2E1 mRNAs were increased in F48 and F48/R6 rats. Recombinant CYP2C11 metabolized arachidonic acid primarily to epoxides with preference for the 14(S),15(R)-, 11(R), 12(S)-, and 8(S),9(R)- epoxyeicosatrienoic acid enantiomers. We conclude that (1) nutritional status affects hepatic microsomal arachidonic acid metabolism, (2) reduced epoxygenase activity in F48 and F48/R6 rats is accompanied by decreased levels of CYP2C11, (3) increased omega-1 hydroxylase activity is accompanied by augmented levels of CYP2E1, and (4) the effects of fasting on CYP2C11 and CYP2E1 expression occur at the pretranslational level.

Cyclosporine suppresses rat hepatic cytochrome P450 in a time-dependent manner

BACKGROUND

Cyclosporine is a potent immunosuppressant know to selectively suppress specific cytochrome P450 (P450) isoforms following chronic therapy in the rat. Cyclosporine undergoes significant hepatic metabolism in the rat, primarily due to P450 3A isoforms. Hence, alterations in hepatic metabolism of cyclosporine may lead to changes in drug pharmacokinetics or pharmacodynamics. The purpose of this study was to examine the temporal effect of chronic cyclosporine dosing on P450 protein expression and metabolic activity in a rat model of chronic cyclosporine nephropathy.

METHODS

Adult male rats were administered cyclosporine 15 mg/kg/day or vehicle 1 ml/kg/day by subcutaneous injection for up to 28 days. To examine whether or not metabolic activity recovered following drug removal, additional rats were administered cyclosporine for 28 days followed by vehicle for up to an additional 15 days. Hepatic P450 protein expression and microsomal metabolic activity were measured by Western blot analysis and in vitro steroid hydroxylation, respectively.

RESULTS

Cyclosporine trough levels progressively increased over the 28 days period and were still measurable for up to 15 days after discontinuation. Immunoblot analysis indicated that chronic cyclosporine treatment suppressed P450 3A2 expression and in vitro steroid hydroxylation in a time-dependent manner. Fifteen days following discontinuation of cyclosporine dosing, hepatic metabolic activity and microsomal P450 3A2 levels returned to near pre-dosing levels.

CONCLUSIONS

We conclude that the time-dependent P450 suppression by cyclosporine may at least partially explain the variability in cyclosporine pharmacokinetics. These studies support the hypothesis that hepatic isoforms other than P450 3A2 may be responsible for cyclosporine metabolism during chronic treatment in the rat.

Arachidonic acid metabolism in the marine fish Stenotomus chrysops (Scup) and the effects of cytochrome P450 1A inducers

Cytochrome P450-mediated arachidonic acid (AA) metabolism was investigated in the marine fish scup, Stenotomus chrysops. Liver microsomes incubated with AA and NADPH produced epoxyeicosatrienoic acids (EETs) and their hydration products (dihydroxyeicosatrienoic acids, DHETs), midchain conjugated dienols (midchain HETEs), and C16-through C20-alcohols of AA (omega-terminal HETEs), all identified by HPLC and GC/MS. Gravid females had 4-fold lower AA metabolism rates than males but identical metabolite profiles. The 5,6-EET (inferred from stable metabolites) was most abundant (47% of total EETs) followed by 14,15-, 11,12-, and 8,9-EET (27, 13, and 13%, respectively). The 12-HETE represented 25% of total HETEs followed in abundance by 16-, 15-, 11-, 19-, 20-, 8-, and 9-HETE. Antibodies against scup CYP1A and a scup CYP2B-like protein inhibited liver microsomal AA metabolism by 30 and 46%, respectively. GC/MS analysis revealed EETs and DHETs as endogenous constituents in scup liver; the predominant EETs were 8,9- and 14,15-EET, followed by a lesser amount of 11,12-EET. Chiral analysis showed a preference for the S,R-enantiomers of endogenous 8,9-, 11,12-, and 14,15-EET (optical purities 80, 64, and 64%, respectively). Treatment of scup with the CYP1A inducer benzo(a)pyrene (BP) increased liver microsomal formation of EETs and HETEs by 2.7-fold in spring and 1.7-fold in summer. BP treatment did not affect microsomal EET regioselectivity, but shifted hydroxylation in favor of 19-HETE and induced 17-HETE formation. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) treatment in summer did not induce liver microsomal AA metabolism rates, yet BP and TCDD both increased endogenous EET content of liver (5- and 3-fold, respectively), with a shift to 14,15-EET. BP treatment increased the selectivity for the S,R-enantiomers of endogenous 8,9-, 11,12-, and 14,15-EET (optical purities 91, 84, and 83%, respectively). Kidney, gill, and heart microsomes all metabolized AA, at rates 10- to 30-fold less than liver microsomes. Similar amounts of endogenous 8,9- and 14,15-EET and less 11,12-EET were detected in heart and kidney, and there was a strong enantioselectivity for 8(R),9(S)-EET in heart (optical purity 78%) but not in kidney. BP treatment did not alter the total EET content in these organs but did shift the regiochemical profile in heart to favor 14,15-EET. Thus, scup liver and extrahepatic organs metabolize AA via multiple cytochrome P450 (CYP) forms to eicosanoids in vitro and in vivo. BP or TCDD induced endogenous AA metabolism in liver, altering EET regioselectivity and, with BP, stereoselectivity. While AhR agonists alter metabolism of AA in early diverging vertebrates expressing both CYP1A and AhR, the magnitude of effects may depend upon the type of inducer.

Salt sensitivity: concept and pathogenesis

Almost two decades ago, the existence of a subset of essential hypertensive patients, who were sensitive (according to the increase in blood pressure levels) to the intake of a diet with a high salt content, was described. These patients are characterized by an increase in blood pressure and in body weight when switched from a low to a high sodium intake. The increase in body weight is due to the incapacity of the kidneys to excrete the whole intake of sodium until renal perfusion pressure (mean blood pressure) attains a level that is able to restore pressure-natriuresis relationship to values that enable the kidney to excrete the salt ingested or administered intravenously. Salt sensitivity does not seem to depend on the existence of an intrinsic renal defect to handle sodium, but on the existence of subtle abnormalities in the regulation of the sympathetic nervous system, the renin-angiotensin system or endothelial function. It is also relevant that organ damage secondary to arterial hypertension, has been shown in animal models and in hypertensive humans sensitive to a high salt intake to be significantly higher when compared with that of salt-resistant animals or humans. Interestingly, in humans, salt sensitivity has been shown to correlate with microalbuminuria, an important predictor of cardiovascular morbidity and mortality, which correlates with most of the cardiovascular risk factors commonly associated with arterial hypertension. One of these factors is insulin resistance, that usually accompanies high blood pressure in overweight and obese hypertensives. Insulin resistance and hyperinsulinism are present in a significant percentage of hypertensive patients developing cardiovascular symptoms or death. For these reasons, therapy of arterial hypertension must be directed, not only to facilitate the lowering of BP level, but also, to halt the mechanisms underlying the increase in BP, when salt intake is increased. Furthermore, therapy must preferably improve the diminished insulin sensitivity present in salt-sensitive subjects that contribute independently to increased cardiovascular risk.

Molecular cloning, expression, and functional significance of a cytochrome P450 highly expressed in rat heart myocytes

A cDNA encoding a P450 monooxygenase was amplified from reverse transcribed rat heart and liver total RNA by polymerase chain reaction using primers based on the 5'- and 3'-end sequences of two rat pseudogenes, CYP2J3P1 and CYP2J3P2. Sequence analysis revealed that this 1,778-base pair cDNA contained an open reading frame and encoded a new 502 amino acid protein designated CYP2J3. Based on the deduced amino acid sequence, CYP2J3 was approximately 70% homologous to both human CYP2J2 and rabbit CYP2J1. Recombinant CYP2J3 protein was co-expressed with NADPH-cytochrome P450 oxidoreductase in Sf9 insect cells using a baculovirus expression system. Microsomal fractions of CYP2J3/NADPH-cytochrome P450 oxidoreductase-transfected cells metabolized arachidonic acid to 14,15-, 11,12-, and 8, 9-epoxyeicosatrienoic acids and 19-hydroxyeicosatetraenoic acid as the principal reaction products (catalytic turnover, 0.2 nmol of product/nmol of cytochrome P450/min at 37 degrees C). Immunoblotting of microsomal fractions prepared from rat tissues using a polyclonal antibody raised against recombinant CYP2J2 that cross-reacted with CYP2J3 but not with other known rat P450s demonstrated abundant expression of CYP2J3 protein in heart and liver. Immunohistochemical staining of formalin-fixed paraffin-embedded rat heart tissue sections using the anti-CYP2J2 IgG and avidin-biotin-peroxidase detection localized expression of CYP2J3 primarily to atrial and ventricular myocytes. In an isolated-perfused rat heart model, 20 min of global ischemia followed by 40 min of reflow resulted in recovery of only 44 +/- 6% of base-line contractile function. The addition of 5 microM 11, 12-epoxyeicosatrienoic acid to the perfusate prior to global ischemia resulted in a significant 1.6-fold improvement in recovery of cardiac contractility (69 +/- 5% of base line, p = 0.01 versus vehicle alone). Importantly, neither 14,15-epoxyeicosatrienoic acid nor 19-hydroxyeicosatetraenoic acid significantly improved functional recovery following global ischemia, demonstrating the specificity of the biological effect for the 11, 12-epoxyeicosatrienoic acid regioisomer. Based on these data, we conclude that (a) CYP2J3 is one of the predominant enzymes responsible for the oxidation of endogenous arachidonic acid pools in rat heart myocytes and (b) 11,12-epoxyeicosatrienoic acid may play an important functional role in the response of the heart to ischemia.

Dietary salt increases first-pass elimination of oral quinidine

BACKGROUND

Some cytochrome P450 (CYP) enzymes, including CYP3A, are expressed not only in the liver but also in the intestine; the latter may therefore be an important site of drug disposition. Animal data suggests that dietary salt modulates expression of renal CYPs. We therefore hypothesized that intestinal CYP3A may be similarly modulated by dietary salt.

METHODS

The effect of changes in dietary salt on the disposition of two CYP3A substrates, quinidine (administered orally and intravenously) and 14C-erythromycin (administered intravenously) were determined after normal volunteers were given high-salt (400 mEq/day) and low-salt (10 mEq/day) diets for 7 to 10 days each.

RESULTS

Plasma concentrations after oral quinidine were significantly lower during the high-salt phase, with the difference between the two treatments attributable to changes within the first 1 to 4 hours after administration. For example, the area under the plasma concentration-time curve for the first hour after drug administration was 0.56 +/- 0.38 microgram.hr/ml for the high-salt diet compared with 1.57 +/- 0.60 micrograms.hr/ml for the low-salt diet (p < 0.05). Similarly, the peak plasma concentration (Cmax) achieved was lower and the time to reach Cmax was later for the high-salt diet (p < 0.05). In contrast, the terminal phase elimination half-lives were similar for the two diets, and no differences in disposition were found with the intravenous drug. The erythromycin breath test was unaffected by the dietary treatments.

CONCLUSIONS

These results indicate an effect of dietary salt on the presystemic disposition of orally administered quinidine. Although the mechanism(s) of CYP3A activity modulation is unknown, this finding may be important in determining drug availability in conditions associated with abnormal salt homeostasis.

Predominant expression of an arachidonate epoxygenase in islets of Langerhans cells in human and rat pancreas

Our laboratory recently described a new human cytochrome P450 arachidonic acid epoxygenase (CYP2J2) and the corresponding rat homolog (CYP2J3). Immunoblotting studies using a polyclonal antibody raised against recombinant human CYP2J2 confirmed CYP2J protein expression in human and rat pancreatic tissues. Immunohistochemical staining of formalin-fixed paraffin-embedded rat and human pancreas using the anti-CYP2J2 IgG and avidin-biotin-peroxidase detection revealed that CYP2J2 protein expression was highly localized to cells in the islets of Langerhans, with minimal staining in pancreatic exocrine cells. Colocalization studies using antibodies to the glucagon, insulin, somatostatin, and pancreatic polypeptide as markers for alpha-, beta-, delta-, and PP cells, respectively, showed that CYP2J protein expression was abundantly present in all four cell types, but was highest in the glucagon-producing alpha-cells. Direct evidence for the epoxidation of arachidonic acid by pancreatic cytochrome P450 was provided by documenting, for the first time, the presence of epoxyeicosatrienoic acids in vivo in human and rat pancreas by gas chromatography/mass spectrometry. Importantly, the levels of immunoreactive CYP2J2 in different human pancreatic tissues were highly correlated with endogenous epoxyeicosatrienoic acid concentrations. We conclude that human and rat pancreas contain an arachidonic acid epoxygenase belonging to the CYP2J subfamily that is highly localized to islet cells. These data together with previous work showing effects of epoxyeicosatrienoic acids in stimulating insulin and glucagon secretion from isolated rat pancreatic islets support the hypothesis that epoxygenase products may be involved in stimulus-secretion coupling in the pancreas.

Inhibition of renal outer medullary 20-HETE production produces hypertension in Lewis rats

Recent studies have indicated that a deficiency in the production of 20-hydroxyeicosatetraenoic acid (20-HETE) in the outer medulla of the kidney may contribute to the abnormalities in the renal handling of sodium and the development of hypertension in Dahl salt-sensitive rats. To determine whether a reduction in 20-HETE production in the outer medulla is sufficient to induce hypertension, an inhibitor of the renal metabolism of arachidonic acid by P450 enzymes, 17-octadecenoic acid (17-ODYA), was chronically infused directly into the outer medulla of the left kidney of uninephrectomized Lewis rats fed a high salt diet. Renal medullary interstitial infusion of 17-ODYA (400 pmol/min) reduced the formation of 20-HETE in the outer medulla of the infused kidney by 70% compared with values seen in the right kidney collected when the rat was uninephrectomized, but it had no effect on the production of 20-HETE in the renal cortex. After 5 days, mean arterial pressure rose from 115 +/- 2 to 142 +/- 2 mm Hg (n = 6) in the rats infused with 17-ODYA, while mean arterial pressure was not significantly altered in the rats infused with vehicle alone (116 +/- 1 versus 117 +/- 2 mm Hg, n = 6). These results suggest that inhibition of the renal metabolism of arachidonic acid by P450 enzymes in the outer medulla of the kidney is sufficient to induce the development of hypertension in Lewis rats fed a high salt diet and support the view that P450 metabolites of arachidonic acid play an important role in the regulation of renal function and the long-term control of arterial pressure.

Bisallylic hydroxylation and epoxidation of polyunsaturated fatty acids by cytochrome P450

Polyunsaturated fatty acids can be oxygenated by cytochrome P450 to hydroxy and epoxy fatty acids. Two major classes of hydroxy fatty acids are formed by hydroxylation of the omega-side chain and by hydroxylation of bisallylic methylene carbons. Bisallylic cytochrome P450-hydroxylases transform linoleic acid to 11-hydroxylinoleic acid, arachidonic acid to 13-hydroxyeicosa-5Z,8Z,11Z,14Z-tetraenoic acid, 10-hydroxyeicosa-5Z,8Z,11Z,14Z-tetraenoic acid and 7-hydroxyeicosa-5Z,8Z,11Z,14Z-tetraenoic acid and eicosapentaenoic acid to 16-hydroxyeicosa-5Z,8Z,11Z,14Z,17Z-pent aenoic acid, 13-hydroxyeicosa-5Z,8Z,11Z,14Z,17Z-pent aenoic acid and 10-hydroxyeicosa-5Z,8Z,11Z,14Z,17Z-pent aenoic acid as major metabolites. The bisallylic hydroxy fatty acids are chemically unstable and decompose rapidly to cis-trans conjugated hydroxy fatty acids during acidic extractive isolation. Bisallylic hydroxylase activity appears to be augmented in microsomes induced by the synthetic glucocorticoid dexamethasone and by some other agents, but the P450 gene families of these hydroxylases have yet to be determined. The fatty acid epoxides, which are formed by cytochrome P450, are chemically stable, but are hydrolyzed to diols by soluble epoxide hydrolases. Epoxidation of polyunsaturated fatty acids is a prominent pathway of metabolism in the liver and the renal cortex and epoxy-genase activity appears to be under homeostatic control in the kidney. Many arachidonate epoxygenases have been identified belonging to the CYP2C gene subfamily. Epoxygenases have also been found in the central nervous system, endocrine organs, the heart and endothelial cells. Epoxides of arachidonic acid have been found to exert pharmacological effects on many cells.

The highly stereoselective oxidation of polyunsaturated fatty acids by cytochrome P450BM-3

Cytochrome P450BM-3 catalyzes NADPH-dependent metabolism of arachidonic acid to nearly enantiomerically pure 18(R)-hydroxyeicosatetraenoic acid and 14(S), 15(R)-epoxyeicosatrienoic acid (80 and 20% of total products, respectively). P450BM-3 oxidizes arachidonic acid with a rate of 3.2 +/- 0.4 micromol/min/nmol at 30 degrees C, the fastest ever reported for an NADPH-dependent, P450-catalyzed reaction. Fatty acid, oxygen, and NADPH are utilized in an approximately 1:1:1 molar ratio, demonstrating efficient coupling of electron transport to monooxygenation. Eicosapentaenoic and eicosatrienoic acids, two arachidonic acid analogs that differ in the properties of the C-15-C-18 carbons, are also actively metabolized by P450BM-3 (1.4 +/- 0.2 and 2.9 +/- 0.1 micromol/min/nmol at 30 degrees C, respectively). While the 17,18-olefinic bond of eicosapentaenoic acid is epoxidized with nearly absolute regio- and stereochemical selectivity to 17(S),18(R)-epoxyeicosatetraenoic acid (>/=99% of total products, 97% optical purity), P450BM-3 is only moderately regioselective during hydroxylation of the eicosatrienoic acid omega-1, omega-2, and omega-3 sp3 carbons, with 17-, 18-, and 19-hydroxyeicosatrienoic acid formed in a ratio of 2.4:2.2:1, respectively. Based on the above and on a model of arachidonic acid-bound P450BM-3, we propose: 1) the formation by P450BM-3 of a single oxidant species capable of olefinic bond epoxidation and sp3 carbon hydroxylation and 2) that product chemistry and, thus, catalytic outcome are critically dependent on active site spatial coordinates responsible for substrate binding and productive orientation between heme-bound active oxygen and acceptor carbon bond(s).

Arachidonic acid diols produced by cytochrome P-450 monooxygenases are incorporated into phospholipids of vascular endothelial cells

Epoxyeicosatrienoic acids (EETs) are synthesized by cytochrome P-450 monooxygenases and released into the blood. When taken up by vascular endothelial and smooth muscle cells, the EETs are primarily esterified to phospholipids or converted to dihydroxyeicosatetraenoic acids (DHETs) and released. In the present studies, radiolabeled 8,9-, 11,12-, and 14,15-DHETs released into the medium from vascular smooth muscle cells were isolated and incubated for 4-16 h with cultured bovine aortic endothelial cells. The uptake ranged from 2 to 50% for the three regioisomers. Hydrolysis of the endothelial lipids and gas chromatographic-mass spectral analyses of the products indicated that all three DHET regioisomers were incorporated intact into phosphatidylcholine and phosphatidylinositol. Similar incubations with EETs confirmed that small amounts of DHETs were also esterified to endothelial phospholipids. These studies indicate that DHETs are incorporated into phospholipids either at the time of EET conversion to DHET or upon release and re-uptake of DHETs. Beside demonstrating for the first time that fatty acid diols are incorporated intact into endothelial lipids, these studies raise the possibility that both EETs and DHETs remain long enough in the vascular wall to produce chronic vasoactive effects.

Renal cytochrome P4504A activity and salt sensitivity in spontaneously hypertensive rats

Differences in the renal metabolism of arachidonic acid by cytochrome P450 have been reported in the spontaneously hypertensive rat (SHR) and Wistar-Kyoto rats, but the contribution of this system to the development of hypertension is unclear. The present study compared renal P450 activity and blood pressure in SHR and Brown-Norway rats (BN) under control conditions and in response to an elevation in sodium intake; genetic linkage analysis was performed in an F2 population (n=219) derived from these strains. Basal renal P4504A enzyme activity measured by conversion of [C(14)]arachidonic acid to 20-hydroxyeicosatetraenoic acid (20-HETE) was significantly greater in the kidneys of adult SHR (n=7) than of BN (n=8) (82 +/- 7 versus 60 +/- 5 pmol/min per milligram protein). Renal 20-HETE production fell 45 percent in SHR and 22 percent in BN in which salt intake was elevated by drinking of saline instead of water for 2 weeks. Mean arterial pressure averaged 157 +/- 3mm Hg in SHR (n = 9) and 100 +/- 2 mm Hg in BN fed a normal salt diet, and it rose to 170 +/- 7 mm Hg (P<.05) in SHR and fell to 90 +/- 3 mm Hg (P<.05) in BN (n=8) after sodium intake was elevated. A polymorphic marker, D5Rjr1, that spanned a repeated element in the P4504A gene on chromosome 5, where all three P4504A isoforms are located, was used for genotyping of the F2 population. The P4504A genotype did not cosegregate with baseline mean arterial pressure in the F2 population; however, significant linkage was observed with the change in mean arterial pressure after sodium intake of the rats was elevated. The degree of linkage differed in male and female rats, and the highest LOD score (3.6) was observed in male F2 rats with a BN grandfather. These findings suggest that the difference in renal P450 activity in SHR and BN does not contribute to the development of hypertension in this F2 population, but it may play some role in determining the blood pressure response to an elevation in salt intake.

Introduction to eicosanoids and the gastroenteric tract

Eicosanoids are produced throughout the gastrointestinal tract and are significant mediators of physiologic and pathophysiologic processes. Understanding the precise role(s) of specific eicosanoid metabolites remains a significant challenge, but has led to the development of new pharmacologic strategies for treating NSAID-induced gastroenteropathy and IBD. Given the complex array of arachidonic acid metabolites, the development of more specific and potent inhibitors of these cyclooxygenase isoforms is important for future studies and possible therapeutic applications. Mice have been prepared that lack expression of COX-1 or COX-2. Once these animals have been carefully evaluated, understanding of the role of various pathways of eicosanoid formation in gastrointestinal function, development, and epithelial growth regulation might be improved. Considerable progress has been made in the understanding of arachidonic acid metabolism and in eicosanoid receptor biology. The identification and characterization of an inducible cyclooxygenase isoform has led to important studies evaluating the role of this enzyme in inflammation, neoplasia, and NSAID-induced gastrointestinal injury. The demonstration that COX-2 overexpression in intestinal epithelial cells leads to specific phenotypic changes, such as increased adhesion and inhibition of apoptosis, indicates that this enzyme may alter the tumorigenic potential of epithelial cells and offers hope for the future development of improved chemopreventive agents.

Molecular cloning and expression of CYP2J2, a human cytochrome P450 arachidonic acid epoxygenase highly expressed in heart

A cDNA encoding a human cytochrome P450 arachidonic acid epoxygenase was isolated from a human liver cDNA library. Sequence analysis revealed that this 1,876-base pair cDNA contained an open reading frame and encoded a new 502-amino acid protein designated CYP2J2. Blot hybridization analysis of RNA prepared from human tissues revealed that CYP2J2 was highly expressed in the heart. Recombinant CYP2J2 protein was prepared using the baculovirus expression system and purified to near electrophoretic homogeneity. The enzyme metabolized arachidonic acid predominantly via olefin epoxidation to all four regioisomeric cis-epoxyeicosatrienoic acids (catalytic turnover 65 pmol of product formed/nmol of cytochrome P450/min at 30 degrees C). Epoxidation of arachidonic acid by CYP2J2 at the 14,15-olefin was highly enantioselective for (14R, 15S)-epoxyeicosatrienoic acid (76% optical purity). Immunoblotting of microsomal fractions prepared from human tissues using a polyclonal antibody raised against the recombinant hemoprotein confirmed primary expression of CYP2J2 protein in human heart. The in vivo significance of CYP2J2 was suggested by documenting the presence of epoxyeicosatrienoic acids in the human heart using gas chromatography/mass spectroscopy. Importantly, the chirality of CYP2J2 products matched that of the epoxyeicosatrienoic acid enantiomers present, in vivo, in human heart. We propose that CYP2J2 is one of the enzymes responsible for epoxidation of endogenous arachidonic acid pools in human heart and that epoxyeicosatrienoic acids may, therefore, play important functional roles in cardiac physiology.