Cytochrome P450-derived arachidonic acid metabolism in the rat kidney: characterization of selective inhibitors

We characterized the inhibitory activity of several acetylenic and olefinic compounds on cytochrome P450 (CYP)-derived arachidonic acid omega-hydroxylation and epoxidation using rat renal cortical microsomes and recombinant CYP proteins. Among the acetylenic compounds, 6-(2-propargyloxyphenyl)hexanoic acid (PPOH) and N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide were found to be potent and selective inhibitors of microsomal epoxidation with IC50 values of 9 and 13 microM, respectively. On the other hand, 17-octadecynoic acid inhibited both omega-hydroxylation and epoxidation of arachidonic acid with IC50 values of 7 and 5 microM, respectively. The olefinic compounds N-methylsulfonyl-12, 12-dibromododec-11-enamide (DDMS) and 12, 12-dibromododec-11-enoic acid (DBDD) exhibited a high degree of selectivity inhibiting microsomal omega-hydroxylation with an IC50 value of 2 microM, whereas the IC50 values for epoxidation were 60 and 51 microM for DDMS and DBDD, respectively. Studies using recombinant rat CYP4A isoforms showed that PPOH caused a concentration-dependent inhibition of omega-hydroxylation and 11, 12-epoxidation by CYP4A3 or CYP4A2 but had no effect on CYP4A1-catalyzed omega-hydroxylase activity. On the other hand, DDMS inhibited both CYP4A1- and CYP4A3- or CYP4A2-catalyzed arachidonic acid oxidations. Inhibition of microsomal activity by PPOH, but not DDMS, was time- and NADPH-dependent, a result characteristic of a mechanism-based irreversible inhibitor. These studies provide information useful for evaluating the role of the CYP-derived arachidonic acid metabolites in the regulation of renal function and blood pressure.

Alkali burn-induced synthesis of inflammatory eicosanoids in rabbit corneal epithelium

PURPOSE

Alkali burning of the rabbit cornea is a well-established model for the study of anterior surface inflammation, neovascularization, and wound-healing processes. 12-hydroxyeicosanoids have been implicated as mediators of such responses. 12(S)-hydroxyeicosatetraenoic acid (12[S]-HETE) is a lipoxygenase-derived arachidonate metabolite and 12(R)-hydroxyeicosatetraenoic acid (12[R]-HETE) is formed by a cytochrome P450 monooxygenase; both give rise to the potent angiogenic factor 12(R)-hydroxyeicosatrienoic acid (12[R]-HETrE). In this study, the authors correlate the pattern of their synthesis in the corneal epithelium with the inflammatory response after alkali injury.

METHODS

New Zealand albino rabbits were anesthetized and alkali burns created with 10-mm filter paper discs (1 N NaOH for 2 minutes). Corneas were then rinsed; 1 to 7 days later, corneal epithelium was scraped and used to assess 14C-arachidonic acid conversion to 12-HETE and 12-HETrE enantiomers in the presence of NADPH by chiral high-pressure liquid chromatography. The inflammatory response secondary to the alkali burn was quantified through area measurements of reepithelialization and neovascularization.

RESULTS

Alkali burn induced a time-dependent production of corneal epithelial 12-HETE and 12-HETrE. A marked increase in 12-HETE and 12-HETrE synthesis was evident at day 2 (from 22 +/- 7 to 139 +/- 22 ng/hour) after injury, increasing to 800 +/- 68 ng/hour at day 7. Chiral analysis revealed a time-dependent synthesis of the R and S enantiomers of 12-HETE (24% R, 76% S) and 12-HETrE (72% R, 28% S). Total arachidonate metabolism, as well as the formation of 12(R)-HETrE, correlated with the area of neovascularization (P < 0.01 and P < 0.02, respectively).

CONCLUSIONS

The results demonstrate that surviving and regenerating epithelium has an increased capacity of synthesizing 12(S)-HETE and 12(R)-HETE and that maximal production of 12(R)-HETrE, a known direct and indirect angiogenic factor, coincides with neovascularization in this model. Thus, the lipoxygenase and cytochrome P450-dependent activities increased in a time-dependent manner, indicating the potential involvement of both pathways in the inflammatory response to alkali burn. The formation of significant quantities of 12(R)-HETE and 12(R)-HETrE is a novel finding in this alkali injury model.

High affinity binding sites for 12(R)-Hydroxyeicosatrienoic acid [12(R)-HETrE] in microvessel endothelial cells

12(R)-HETrE is an NADPH-dependent arachidonic acid-derived metabolite whose synthesis is induced several fold in inflamed corneal epithelium correlating with the development of the in situ inflammatory response, i.e., vasodilation, PMN chemotaxis, endothelial cell mitogenesis, and neovascularization. Because this novel eicosanoid may serve as an endogenous mediator of the angiogenic response in the cornea during inflammation we probed microvessel endothelial cells for a specific binding site which could possibly account for the mechanism by which this eicosanoid initiates changes in cellular activity. Binding of radioactive ligand [3H-12(R)-HETrE] was saturable with time and concentration. Scatchard analysis indicated a single, saturable binding site for 12(R)-HETrE with a Bmax = 24,700 sites/cell and an apparent Kd = 0.043 nM. Thin layer chromatography analysis of cell-associated ligand revealed that esterification of 12(R)-HETrE was 2-7 fold less than unesterified, cell bound ligand. The concentrations of 12(R)-HETrE at which maximum biological activity has been observed, i.e., 0.1 nM, roughly corresponds to the Kd value, suggesting a functional link to this binding site. These studies begin to reveal a potential mechanism by which 12(R)-HETrE stimulates microvessel endothelial cells to invade the cornea leading to corneal neovascularization.

Overexpression of the heme oxygenase gene in renal cell carcinoma

Heme oxygenase (HO) activity has been implicated in the regulation of renal function and cell growth in normal and disease states. Expression of HO genes has been shown to regulate important hemoprotein(s) such as cytochrome P450. In the present study, HO activity was measured in samples of human adenocarcinoma, juxtatumor, and normal renal tissues. The samples were histologically examined to verify the malignant and normal nature. HO activity was 4-fold higher in the adenocarcinoma than in either normal or juxtatumor tissues. We designed a reverse transcriptase-polymerase chain reaction (RT-PCR) method to assess the presence of HO-1 and HO-2 mRNA in biopsy samples of various human renal tissues. Total RNA from renal samples was reverse transcribed and amplified simultaneously by PCR using specific primers for HO-1 and HO-2. Results show that both HO-1 and HO-2 mRNAs were expressed in all renal tissues examined and that HO-1 appeared to be amplified more than HO-2. Northern blot analysis revealed that HO-1 mRNA was elevated by several-fold in adenocarcinoma compared with juxtatumor or normal tissues. In contrast, no differences in HO-2 mRNA levels were observed using either RT-PCR or Northern blot. Cytochrome P450 arachidonic acid epoxygenase and omega-hydroxylase activities were markedly reduced in the tumor tissues, whereas, in the juxtatumor tissue, cytochrome P450 omega-hydroxylase activity was significantly increased. Northern blot analysis using cytochrome P450 cDNA probe 4A2 cDNA for the omega-hydroxylase gene family revealed that mRNA levels for omega-hydroxylase transcripts were significantly decreased in the adenocarcinoma compared with juxtatumor. The decrease in cytochrome P450 4All mRNA levels correlated with a decrease in the arachidonic acid omega-hydroxylation metabolite, 20-HETE. The production of 20-HETE was significantly higher in juxtatumor in agreement with omega-hydroxylase mRNA. Higher levels of HO-1 may be a contributing factor for the undetectable levels of cytochrome P450 arachidonic acid metabolites, 20-HETE, in the adenocarcinoma. Our results suggest that increased generation of mitogenic activities by omega-hydroxylase and 20-HETE in the juxtatumor may be a contributing factor in the development and growth of neoplastic tissues, and the induction of HO in the tumor tissue may be an attempt to limit oxidative injury caused by the cytochrome P450 metabolites and other oxidative stress.

Cloning, sequencing, and cDNA-directed expression of the rat renal CYP4A2: arachidonic acid omega-hydroxylation and 11,12-epoxidation by CYP4A2 protein

20-Hydroxy-5,8,11,14-eicosatetraenoic acid (20-HETE), the omega-hydroxylation product of arachidonic acid, is the major metabolite produced in the kidney. It has potent biological effects on renal tubular and vascular functions and on the long-term control of arterial pressure. The synthesis of 20-HETE is catalyzed by enzymes of the CYP4A family, among which CYP4A2 is the most abundant isozyme expressed in the kidneys of rats. We have cloned and sequenced the CYP4A2 cDNA from the kidney of Lewis-Wistar rats and directed its expression using baculovirus and Sf9 insect cells. A high level of expression of CYP4A2 was evident by Northern, Western, and spectral analyses revealing a P450 content of 0.3 nmol/mg microsomal protein. To study CYP4A2-catalyzed arachidonic acid omega-hydroxylation, Sf9 cells were coinfected with CYP4A2 and NADPH cytochrome P450 oxidoreductase (OR) recombinant viruses. CYP4A2/OR membranes metabolized lauric acid at a high rate (7 and 5.5 nmol/min/nmol P450 in the presence and absence of b5, respectively). However, arachidonic acid omega-hydroxylase activity was barely detectable. When purified OR was added to the membranes expressing CYP4A2 protein, a concentration-dependent production of 20-HETE was observed. Maximal synthesis of 20-HETE of 0.89 nmol/min/nmol P450 was achieved at OR:CYP4A2 ratio of 14:1. The omega-hydroxylation of arachidonic acid was dependent on the presence of b5. Furthermore, increasing OR concentrations yielded additional arachidonic acid metabolite identified by GC/MS as 11,12-EET. Microsomes prepared from isolated renal microvessels selectively expressed CYP4A2 protein and readily metabolized arachidonic acid to two major metabolites, 20-HETE and 11,12-DHET, the hydrolytic metabolite of 11, 12-EET. It is suggested that CYP4A2 functions as the renal microvessel arachidonate omega-hydroxylase and that it can also catalyze the 11,12-epoxidation of arachidonic acid.

Cytochrome P450 4A expression and arachidonic acid omega-hydroxylation in the kidney of the spontaneously hypertensive rat

20-Hydroxyeicosatetraenoic acid (20-HETE) is a major arachidonate metabolite in the kidney of the spontaneously hypertensive rat (SHR). The increase in its synthesis has been associated with the elevation of blood pressure in the SHR. The omega-hydroxylation of arachidonic acid is an activity associated with members of the CYP4A gene family which, in the rat, comprises three major isoforms: 4A1, 4A2 and 4A3. 20-HETE displays potent and diverse biological activities which can affect pro- and anti-hypertensive mechanisms dependent upon where, when and by which isoform it has been produced. Therefore, it is important to identify and characterize its biosynthetic system. We compared CYP4A mRNA and protein expression to patterns of 20-HETE synthesis in the SHR kidney. The reverse transcription/polymerase chain reaction (RT/PCR) technique was used to amplify CYP4A mRNA in microdissected nephron segments. Southern blot hybridization of PCR products obtained from nephron segments with the CYP4A1 cDNA probe demonstrated strong signals in S2 and S3 segments of the proximal tubule. Immunoblots of nephron segments using a polyclonal anti-rat liver CYP4A1 antibody which cross-reacts with CYP4A2 and CYP4A3, and 14C-arachidonic acid metabolism, confirmed that arachidonic acid omega-hydroxylation, i.e., 14C-20HETE formation, and CYP4A proteins were also localized mainly in the S2 and S3 segments. Correlation also existed between the age-dependent increase in arachidonate omega-hydroxylation in the kidney and CYP4A mRNA levels as measured by Northern hybridization of total RNA using the CYP4A1 cDNA probe. Immunoblot analysis revealed that at 7 weeks, where 20-HETE production is at its maximum, all three proteins are expressed. CYP4A3 and 4A1 immunoreactive proteins appeared at 3 weeks, showed maximum levels at 5 and 7 weeks, respectively, and gradually decreased to lower levels at 13 and 20 weeks, whereas CYP4A2 levels were undetectable at 3, 5 and 7 weeks but appeared at 13-20 weeks. Additional immunoblots indicated that renal cortical CYP4A1 protein levels were higher in SHR compared to Sprague-Dawley and Wistar-Kyoto rats. The increased levels of CYP4A1-immunoreactive band at 7 weeks corresponded to the maximal activity of arachidonate omega-hydroxylation. Thus, CYP4A1 might play a significant role in contributing to the increased cortical/proximal production of 20-HETE seen in 7-week-old SHR. However, given the high homology among members of the CYP4A gene family and the lack of specific tools to discern among these isoforms, additional studies have to be carried out to substantiate our findings.

Chiral analysis of 12-hydroxyeicosatetraenoic acid formed by calf corneal epithelial microsomes

The production of 12-hydroxyeicosatetraenoic acid by the corneal epithelium of several species has been extensively reported yet the controversy over the exclusive production of the (S) epimer (a lipoxygenase-derived metabolite) endures. Incubation of calf corneal epithelial microsomes (3 mg/ml) with arachidonic acid and NADPH resulted in the formation of 12-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE). The synthesis of 12-HETE was inhibited by SKF-525A and clotrimazole, selective inhibitors of cytochrome P450 dependent activities, but not by indomethacin or BW-755C, inhibitors of cyclooxygenase and lipoxygenase activities, respectively. Chiral analysis revealed the presence of both enantiomers; however, the R isomer was the predominant one, i.e., 91 +/- 5% vs. 9 +/- 5% for the R and S enantiomers, respectively. Since the R enantiomer is the product of a cytochrome P450-mediated reaction, it suggests that the major metabolic activity in these microsomes is cytochrome P450-dependent and supports the claim for cytochrome P450 reactions in this ocular tissue.

20-Hydroxyeicosatetraenoic acid is formed in response to EGF and is a mitogen in rat proximal tubule

Hydroxyeicosatetraenoic acid (20-HETE) is a major cytochrome P-450-arachidonic acid metabolite in the rat kidney, and its synthesis along the nephron is specifically localized to the proximal tubule, where receptor density for epidermal growth factor (EGF) is the highest. EGF stimulated endogenous 20-HETE formation in a concentration and time-dependent manner, i.e., from 1.6 to 2.6 +/- 0.3 and 3.0 +/- 0.6 pmol 20-HETE.mg-1.min-1 at 10(-8) and 10(-7) M EGF, respectively. The effect of 20-HETE on proximal tubular cell proliferation was examined using primary cultures of rat proximal tubular cells and proximal tubular-derived cell lines, LLC-PK1 and opossum kidney OK. In both cell lines, 20-HETE increased thymidine incorporation into DNA with maximal effect at 10(-9) M. Addition of 20-HETE to serum-deprived LLC-PK1 or OK cells for 48 h caused a concentration-dependent increase in cell number with maximal effect at 10(-9) M. This effect was specific, as structurally similar eicosanoids such as 20-COOH-arachidonic acid, 19(R)-HETE, and 19(S)-HETE did not increase cell number. In 4-day primary cultures of proximal tubular cells, EGF (10(-9) M) and 20-HETE (10(-9) M) increased bromodeoxyuridine (BrdU) incorporation by 40 and 28%, respectively. Addition of both resulted in a twofold increase in BrdU incorporation. Although 20-HETE synthesis in cultured cells is greatly diminished with time, significant picomolar concentrations can be obtained in 4-day cultures. Addition of 17-octadecynoic acid (17-ODYA), an inhibitor of 20-HETE synthesis, significantly inhibited EGF-stimulated BrdU incorporation. The demonstrations that EGF stimulates proximal tubular 20-HETE production and that the latter is a potent mitogen to these cells suggests that 20-HETE may act as a mediator of the EGF effect on cellular growth in the proximal tubule.

Adenovirus-mediated heme oxygenase-1 gene transfer into rabbit ocular tissues

PURPOSE

Heme oxygenase-1 (HO-1) is a stress protein induced up to 100-fold within a few hours after exposure to oxidative stress, and it has been shown to counteract oxidative injury induced by ultraviolet light or free radicals. The current study was undertaken to determine whether the HO-1 gene can be introduced into adult rabbit ocular tissues by microinjection of a recombinant replication-deficient adenovirus human HO-1 cDNA (Adv-HHO).

METHODS

Human HO-1 gene was used for transfection studies to differentiate endogenous from transfected HO. The purified Adv-HHO construct (10(8) pfu/ml) was mixed with lipofectamine and microinjected into the anterior chamber, vitreous cavity, and subretinal space of New Zealand rabbit eyes. After 2 weeks, total RNA was extracted from different ocular tissues, reverse transcription-polymerase chain reaction was performed using specific human HO-1 primers, and amplification products were subjected to Southern hybridization.

RESULTS

Transfection with the Adv-HHO construct into rabbit corneal epithelial cells in culture resulted in a functional expression of the human HO-1 gene; the human HO-1 mRNA was detected, and enzyme activity increased threefold. Human HO-1 mRNA was detected in the retina after microinjection of the Adv-HHO construct into the subretinal space. Microinjection into the vitreous resulted in HO-1 mRNA expression in the corneal endothelium, iris, lens, and retina; after intracameral injection of the Adv-HHO construct, human HO-1 mRNA was detected in corneal epithelium and endothelium, ciliary body, lens, and iris. Regardless of the injection site, transfected human HO-1 mRNA was undetectable in tissues outside the eye, that is, brain, liver, and kidney.

CONCLUSIONS

These results demonstrated a tissue-selective functional transfer of the human HO-1 gene into rabbit ocular tissues in vivo. This technique may be a promising means for delivering HO-1 gene in vivo as a protective mechanism against oxidative stress that contributes to the pathogenesis of ocular diseases such as cataract, light-induced injury, age-related macular degeneration, and diabetic retinopathy.

Transfection of the human heme oxygenase gene into rabbit coronary microvessel endothelial cells: protective effect against heme and hemoglobin toxicity

Heme oxygenase (HO) is a stress protein and has been suggested to participate in defense mechanisms against agents that may induce oxidative injury such as metals, endotoxin, heme/hemoglobin, and various cytokines. Overexpression of HO in cells might therefore protect against oxidative stress produced by certain of these agents, specifically heme and hemoglobin, by catalyzing their degradation to bilirubin, which itself has antioxidant properties. We report here the successful in vitro transfection of rabbit coronary microvessel endothelial cells with a functioning gene encoding the human HO enzyme. A plasmid containing the cytomegalovirus promoter and the human HO cDNA complexed to cationic liposomes (Lipofectin) was used to transfect rabbit endothelial cells. Cells transfected with human HO exhibited an approximately 3.0-fold increase in enzyme activity and expressed a severalfold induction of human HO mRNA as compared with endogenous rabbit HO mRNA. Transfected and nontransfected cells expressed factor VIII antigen and exhibited similar acetylated low-density lipoprotein uptake (two important features that characterize endothelial cells) with > 85% of cells staining positive for each marker. Moreover, cells transfected with the human HO gene acquired substantial resistance to toxicity produced by exposure to recombinant hemoglobin and heme as compared with nontransfected cells. The protective effect of HO overexpression against heme/hemoglobin toxicity in endothelial cells shown in these studies provides direct evidence that the inductive response of human HO to such injurious stimuli represents an important tissue adaptive mechanism for moderating the severity of cell damage produced by these blood components.

Enhancement of delayed hypersensitivity inflammatory reactions in guinea pig skin by 12(R)-hydroxy-5,8,14-eicosatrienoic acid

Delayed-type hypersensitivity (DTH) reactions are initiated by sensitized T cells. Their progression is dependent upon the local release of various autacoids, including cytokines and eicosanoids, by T cells, infiltrating inflammatory cells, and resident tissue cells. 12(R)-hydroxy-5,8,14-eicosatrienoic acid [12(R)-HETrE], an eicosanoid produced by skin and cornea, possesses potent proinflammatory properties at picomolar concentrations including vasodilation, increase in membrane permeability, neutrophil chemotaxis, and angiogenesis. Because DTH reactions are associated with many of these same phenomena, we examined the effect of 12(R)-HETrE and related 12-hydroxyeicosanoids on the expression of DTH to purified protein derivative of tuberculin in sensitized guinea pigs. In the absence of purified protein derivative of tuberculin, none of the eicosanoids evoked erythema or edema after intradermal injection at doses up to 100 pmol. When injected together with purified protein derivative of tuberculin, 12(R)-hydroxy-5,8,10,14-eicosatetraenoic acid [12(R)-HETE], but not its enantiomer 12(S)-HETE, significantly inhibited macroscopic expression of delayed reactivity (erythema) only at the highest dose tested, 10 pmol. In contrast, 12(R)-HETrE significantly enhanced expression of DTH at doses between 1 fmol and 1 pmol (50% and 30% increases above control, respectively). Its stereoisomer, 12(S)-HETrE, did not enhance DTH at any tested dose, but was able to block the activity of 12(R)-HETrE when injected simultaneously. Enhancement or inhibition of visible skin responses was not associated with qualitative or quantitative changes in cellular infiltrates at the reaction site. 12(R)-HETrE had no effect on the nonimmunologic inflammatory skin reaction induced by phorbol myristate acetate, suggesting selectivity toward DTH. We conclude that 12(R)-HETrE enhances DTH via a yet to be determined mechanism and that its stereoisomer, 12(S)-HETrE, may be a useful antagonist for studying the inflammatory actions of this eicosanoid.

Cytochrome P450 arachidonic acid omega-hydroxylation in the proximal tubule of the rat kidney

20-Hydroxy-5,8,11,14-eicosatetraenoic acid (20-HETE) is a major cytochrome P450-dependent arachidonate metabolite in the rat kidney. In the present study we characterized the formation of 20-HETE in the proximal tubule, the nephron segment with the highest concentration of cytochrome P450 activities, including P450 arachidonic acid metabolism. Freshly isolated tubules showed a basal formation of 20-HETE, implying that it is an endogenous constituent of the proximal tubule. Conversion of exogenous arachidonic acid to 20-HETE in proximal tubule homogenates was enzymatic and NADPH-dependent (i.e., 0 and 65.5 +/- 1.1 pmol/mg/min in the absence and presence of NADPH, respectively). That its formation was not affected by indomethacin but inhibited following preincubation with 17-ODYA (17-octadecynoic acid) and 7-ER (7-ethoxyresorufin) suggested that a P450 monooxygenase activity was involved in its synthesis. This was further strengthened by the demonstration that antibody raised against the rat cytochrome P450 4A1, a major fatty acid omega-hydroxylase isozyme, inhibited 20-HETE formation, suggesting the involvement of a P450 4A1 or P450 4A1-like activity in this reaction. Pretreatment of rats with clofibrate and dexamethasone, inducers of the P450 4A gene family, yielded a twofold increase in the proximal tubular synthesis of 20-HETE as well as an increase in P450 4A1 mRNA. These results, together with previous demonstrations that 20-HETE vasoconstricts isolated blood vessels, namely, renal microvessels, and affects tubular ion transport, suggest a role for 20-HETE in the regulation of renal vascular tone and transport functions and further stress the importance of understanding the regulation of 20-HETE synthesis in the kidney.

Altered renal P-450 metabolism of arachidonic acid in Dahl salt-sensitive rats

The renal metabolism of arachidonic acid (AA) was compared in male and female prehypertensive Dahl salt-sensitive (SS/Jr) and salt-resistant (SR/Jr) rats maintained on a low- (0.3%) sodium chloride diet. Renal cortical microsomes incubated with AA produced 20-hydroxyeicosatetraenoic acid (20-HETE), 14,15- and 11,12-epoxyeicosatrienoic acids, and a new metabolite of AA, 11,12-epoxy-20-hydroxyeicosatrienoic acid. The production of 20-HETE was similar in cortical microsomes of female SS/Jr and SR/Jr rats maintained on a low-salt diet (72 +/- 5 vs. 66 +/- 3 pmol.min-1.mg protein-1); however, the formation of epoxygenase metabolites was significantly less in SS/Jr than in SR/Jr rats (45 +/- 2 vs. 70 +/- 3 pmol.min-1.mg protein-1). Outer medullary microsomes produced primarily 20-HETE, and the formation of this compound was significantly lower in SS/Jr than in SR/Jr female rats fed a low-salt diet (8 +/- 2 vs. 18 +/- 3 pmol.min-1.mg protein-1). Renal papillary microsomes produced prostaglandin E2 and F2 alpha, and the formation of these compounds was similar in female SS/Jr and SR/Jr rats fed a low-salt diet. Similar differences in the metabolism of AA by P-450 were observed in microsomes prepared from the renal cortex and outer medulla of male SS/Jr and SR/Jr rats. These results indicate that the renal metabolism of AA by P-450 is altered in prehypertensive Dahl SS/Jr rats; however, the functional significance of this system in resetting renal function and in the development of hypertension in this model remains to be established.

Effects of epoxyeicosatrienoic acids on 86Rb uptake in renal epithelial cells

Epoxyeicosatrienoic acids (EETs) are arachidonic acid metabolites formed endogenously via the cytochrome P450 pathway in rat, rabbit, and human kidney. We characterized the effects of the four regioisomeric EETs on ion transport in the renal epithelial cell line, LLC-PK1. Among the EETs, 14,15-EET was the most potent inhibitor of 86Rb uptake. Its effect was concentration-dependent (IC50 = 75 nM) and stereoselective to the 14S, 15R-EET. Experiments measuring 14,15-EET-induced 86Rb uptake inhibition in the presence of inhibitors of Na(+)-K(+)-ATPase activity (ouabain), Na(+)-K(+)-Cl- cotransporter (furosemide), and Na(+)-H+ exchanger (amiloride) suggested that 14,15-EET inhibits ion transport via an amiloride-sensitive mechanism. These results, together with previous reports demonstrating their endogenous production in the kidney, suggest an important role for EETs, specifically 14,15-EET, in the regulation of ion and water reabsorption in the kidney and implicate their function in renal pathophysiology.

Identification of binding sites for transcription factors NF-kappa B and AP-2 in the promoter region of the human heme oxygenase 1 gene

Heme oxygenase (HO) is the rate-limiting enzyme in heme catabolism and its activity is induced by many agents, including its substrate heme, heavy metals, UV radiation, and other injurious oxidant conditions. We examined the presence of several regulatory elements in the promoter region of the human HO-1 gene which could possibly account for its induction in response to diverse agents or influences. Heme treatment increased both HO activity and HO-1 mRNA in the human erythroleukemic cell line K562. Electrophoretic mobility-shift assays of nuclear protein extracts from heme-treated and control cells with specific oligonucleotide probes containing binding sites for known transcription factors, including AP-1, AP-2, Sp1, NF-kappa B, CTF/NF1, TFIID, OKT1, and CREB, and oligonucleotides containing serum-, metal-, and glucocorticoid-responsive elements demonstrated a specific and marked increase in the NF-kappa B and AP-2 transcription factors and, to a lesser extent, an increase in AP-1. No significant increase in other transcription factors over the control, untreated cells was observed. DNase I footprint assays using purified transcription factors revealed the presence of NF-kappa B and AP-2 binding sites in the proximal part of the promoter region of the human HO-1 gene. Moreover, nucleotide sequence analysis of the HO-1 promoter region showed that the protected regions encompassed NF-kappa B and AP-2 consensus binding sites. The presence of regulatory sequences for the binding of transcription factors such as NF-kappa B and AP-2, whose activation is associated with the immediate response of the cell to an injury, may be an indication of the important role which HO-1 may play in defense mechanisms against tissue injury.

Characterization of a 142-bp fragment of the murine c-fos oncogene promoter upstream of the SIF-binding element

We previously reported that in transformed mouse sarcoma cells of spontaneous origin and in revertants transfected with a fos-cat fusion, the 600-bp c-fos promoter region provides chloramphenicol acetyltransferase activity. In the present study, we investigated the binding of transcriptional factor protein(s) to a region (-503 to -361) upstream of the sis (platelet-derived growth factor)-inducible factor (SIF)-binding element. Gel electrophoresis retardation (GER) assay clearly demonstrated the appearance of strong binding activity to a newly described fragment in the 142-bp region studied. Further analysis using synthetic oligodeoxyribonucleotides and GER defined a binding region of 30 bp (AvaI-AvaII) from -503 to -472 that partially overlaps with a region known to bind fos promoter binding site 2 (FBS2). DNase I footprint analysis discovered a novel sequence in the upstream region of the c-fos promoter to which protein(s) in nuclear extracts from various mouse and human cells bind. This factor(s) is not identical to most known transcriptional factors present in the promoter region of nuclear oncogenes. A proximal part of this fragment is very conservative and contains several AP-2-like-binding sites.

Tin-mediated heme oxygenase gene activation and cytochrome P450 arachidonate hydroxylase inhibition in spontaneously hypertensive rats

The effect of SnCl2 on the transcription of the heme oxygenase gene in spontaneously hypertensive rats was examined using cDNA for the rat heme oxygenase (HO-1). An increase in renal HO-1 mRNA levels was observed in response to SnCl2 treatment. Quantitative evaluation by scanning densitometry demonstrated a maximal increase in HO-1 mRNA 24-fold over control at 8 hours after SnCl2 administration. Nuclear runoff assay using isolated renal nuclei from SnCl2-treated rats revealed an active HO-1 gene transcription. Transcription of HO-1 in rat kidney was greatly increased within 3 hours of administration of SnCl2, as evidenced by the level of [alpha 32P]UTP incorporation into nuclear RNA. As a consequence of activation of the HO-1 gene transcription, renal enzyme activity increased eightfold at 16 hours after SnCl2, and reached maximal activity of 16-fold over control at 32 hours after injection. No significant change in cytochrome P450 fatty acid omega-hydroxylase (P450 4A) mRNA was observed after SnCl2 administration. Cytochrome P450-arachidonic acid omega/omega-1 hydroxylase(s) activity (formation of 20- and 19-HETE) was significantly reduced 24 hours after SnCl2 administration and remained lower than the control level 48 and 72 hours after injection. In addition, blood pressure was reduced from 151 +/- 2.5 mm Hg to 133 +/- 2.3 mm Hg after 48 hours of SnCl2 treatment. The reduction in blood pressure preceded natriuresis. It is concluded that SnCl2 induces activation of the HO-1 gene, which is followed by elevation in enzyme activity and a decrease in cytochrome P450-arachidonic acid omega-hydroxylase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of erythropoietin mRNA in the rat kidney by polymerase chain reaction

Erythropoietin (Epo) is a glycoprotein secreted by kidney cells which plays an important role in the regulation of erythropoiesis. Localization of the Epo production by immunohistochemical studies and in situ hybridization has not been definitively established and is still a matter of controversy. Epo and glyceraldehyde 3-dehydrogenase (GAPDH) mRNA levels were determined in total RNA isolated from control and CoCl2-treated rats using a coupled reverse transcriptase/polymerase chain reaction method (RT/PCR). As indicated by the amount of amplification product, Epo mRNA levels were several-fold higher in CoCl2-treated rat kidney. In contrast, GAPDH mRNA levels were similar in control and CoCl2-treated rats. This RT/PCR method was also used to assess the level of Epo and GAPDH mRNA in microdissected nephron segments. All nephron segments tested lacked any detectable levels of Epo mRNA in either control or CoCl2-treated rats. On the other hand, peritubular cells (capillary fraction: afferent/efferent arteriole, vasa recta) were the only cells where the Epo mRNA was detected. Using a specific primer for GAPDH, the RT/PCR method could identify GAPDH mRNA in all microdissected nephron segments where the Epo mRNA was not expressed. Thus, a combination of microdissected nephron segments and RT/PCR enabled us to detect GAPDH mRNA populations in all nephron segments, whereas the failure to detect Epo mRNA in all segments but the capillary fraction, is due to the specific and localized expression of the Epo gene to this fraction.

Effect of metabolic inhibitors on arachidonic acid metabolism in the corneal epithelium: evidence for cytochrome P450-mediated reactions

The corneal epithelium of several species, has the capacity to metabolize arachidonic acid (arachidonic acid) via an NADPH-dependent cytochrome P450 mechanism. The major metabolites are 12-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE) and 12-hydroxy-5,8,14-eicosatrienoic acid (12-HETrE), both of which exist in stereoisomeric configurations. However, the R enantiomers are predominantly produced by this enzyme system and exhibit potent biological activities. 12(R)-HETE inhibits Na-K-ATPase, increases corneal thickness and reduces intraocular pressure. 12(R)-HETrE causes vasodilation, neutrophil chemoattraction and angiogenesis. The formation of these metabolites is unaffected by cyclooxygenase and lipoxygenase inhibitors (indomethacin, diclofenac and BW755C) but inhibited by cytochrome P450 enzyme inhibitors such as carbon monoxide, SKF-525A and clotrimazole. The capacity of the normal corneal epithelium to metabolize arachidonic acid via cytochrome P450 is very low although under certain conditions this enzymatic pathway may become greatly induced. Corneal epithelial hypoxia in response to contact lens wear results in the time-dependent formation of NADPH-cytochrome P450-dependent arachidonate metabolites, 12(R)-HETE and 12(R)-HETrE. Under this condition, metabolite production correlates strongly with the in situ inflammatory response and inhibition of their formation significantly attenuates inflammation. It is evident that the cytochrome P450 arachidonate metabolites should be added to the realm of cyclooxygenase and lipoxygenase-derived eicosanoids as possible inflammatory mediators. Therefore, studies to evaluate eicosanoid involvement in inflammation should examine inhibitors of this pathway in addition to the classically studied non-steroidal antiinflammatory drugs (NSAIDs).

Novel regulatory sites of the human heme oxygenase-1 promoter region

Heme oxygenase is the rate-limiting enzyme in heme catabolism, the activity of which is induced by several agents including its substrate heme, u.v. radiation and other oxidant injurious conditions. Its biological function is to provide a specific regulatory mechanism for control of the level of many heme proteins. Using a computer-assisted approach, we examined the presence of different regulatory elements on the human heme oxygenase-1 promoter region which could possibly be involved in its regulation. We identified several NFkB and AP-2-like binding sites in the 5' untranslated region of the human heme oxygenase gene. DNase I footprint analysis confirmed the presence of these two important transcriptional sites. This study is the first to reveal the presence of NFkB and AP-2 binding sites in the heme oxygenase-1 promoter region and thus suggests a role for these transcriptional factors in the regulation of heme oxygenase-1 expression, a stress and heat shock protein, under injury conditions.

20-hydroxyeicosatetraenoic acid is an endothelium-dependent vasoconstrictor in rabbit arteries

Recently we have demonstrated that 20-hydroxy-5,8,11,14-eicosatetraenoic acid (20-HETE) constricts rat aortic rings and that this effect is cyclooxygenase- and endothelium-dependent. Incubation of 20-HETE with ram seminal vesicles, a rich source of cyclooxygenase, led to the identification of vasoconstrictor metabolites, the 20-hydroxy-endoperoxides. In the present study, we demonstrated differences in the potency of 20-HETE to constrict several arteries. In all blood vessels tested, the 20-HETE contractile effect was cyclooxygenase- and endothelium-dependent. Differences in contractile potency of 20-HETE varied according to the blood vessels; potency being higher in more muscular arteries than elastic ones. Furthermore, 20-HETE was more potent in eliciting vasoconstriction than its precursor, arachidonic acid. We also provide evidence for the generation of 20-hydroxy-endoperoxides from 20-HETE by the endothelial cyclooxygenase. 20-HETE is a major arachidonate metabolite formed by the cytochrome P450 monooxygenases in rat, human and rabbit kidneys. In addition, blood cells such as leukocytes have the ability to produce 20-HETE, suggesting its presence in the circulation. Furthermore, 20-HETE has been shown to inhibit platelet aggregation. Thus, the ability of 20-HETE to modulate vascular tone and platelet function implicates a role for this compound in the regulation of hemostasis.

20-Hydroxyeicosatetraenoic acid is an endogenous vasoconstrictor of canine renal arcuate arteries

Recent studies have indicated that renal arteries can produce 20-hydroxyeicosatetraenoic acid (20-HETE) and suggest the potential involvement of a P450 metabolite of arachidonic acid in the myogenic activation of canine renal arteries. In the present study, the effects of 20-HETE on isolated canine renal arcuate arteries were studied. Administration of 20-HETE to the bath or the lumen at concentrations of 0.01-1 microM produced a graded reduction in the diameter of these vessels. In contrast, 19(R)-HETE was a vasodilator, whereas 19(S)-HETE was relatively inactive. The vasoconstrictor response to 20-HETE was not altered by the cyclooxygenase inhibitor indomethacin, endoperoxide/thromboxane receptor antagonist SQ29548, or combined blockade of the cyclooxygenase, lipoxygenase, and P450 pathways using indomethacin, baicalein, and 7-ethoxyresorufin. The response to 20-HETE was associated with depolarization and a sustained increase in the intracellular calcium concentration in renal vascular smooth muscle cells. Patch-clamp studies indicated that 20-HETE significantly reduced mean open time, the open-state probability, and the frequency of opening of a 117-pS K+ channel recorded from renal vascular smooth muscle cells in the cell-attached mode. Microsomes prepared from the renal cortex of dogs produced 20-HETE and 20-carboxyarachidonic acid when incubated with [14C]arachidonic acid. These results indicate that 20-HETE is an endogenous constrictor of canine renal arcuate arteries. The vasoconstrictor response to 20-HETE resembles the myogenic activation of these vessels after elevations in transmural pressure and suggests a potential role for this substance in the regulation of renal vascular tone.