5,6-Epoxyeicosatrienoic acid mobilizes Ca2+ in anterior pituitary cells

Homology of insect corpora allata and vertebrate adenohypophysis?

Animal species of various phyla possess neuroendocrine glands whose hormonal products regulate developmental and physiological mechanisms and directly impact behavior. Two examples, the corpora allata of insects and the vertebrate adenohypophysis have previously been regarded as analogous tissues that evolved independently from diffuse epidermal nerve nets of early metazoans. More recent developmental and functional studies accumulated evidence suggesting that the bilaterian nervous systems including its modern parts (e.g. pallium or cortex and mushroom bodies) and its neuroendocrine appendages (that are considered to be more ancient structures) possess a single evolutionary origin. The corpora allata of insects and the vertebrate adenohypophysis share a number of characteristics in respect of morphology, control of hormone release by RFamides, metabolites produced by closely related cytochrome P450 enzymes and gene expression during embryonic development. This review incorporates latest findings into an extensive description of similarities between insect corpora allata and vertebrate adenohypophysis that should encourage further studies about the onto- and phylogenetic origin of these neuroendocrine glands.

Epoxyeicosatrienoic acids are endogenous regulators of vasoactive neuropeptide release from trigeminal ganglion neurons

Epoxyeicosatrienoic acids (EETs) are bioactive eicosanoids produced from arachidonic acid by cytochrome P450 epoxygenases. We previously described the expression of cytochrome P450-2J epoxygenase in rat trigeminal ganglion neurons and that EETs signaling is involved in cerebrovascular dilation resulting from perivascular nerve stimulation. In this study, we evaluate the presence of the EETs signaling pathway in trigeminal ganglion neurons and their role in modulating the release of calcitonin gene-related peptide (CGRP) by trigeminal ganglion neurons. Liquid chromatography tandem mass spectrometry identified the presence of each of the four EETs regio-isomers within primary trigeminal ganglion neurons. Stimulation for 1 h with the transient receptor potential vanilloid-1 channel agonist capsaicin (100 nmol/L) or depolarizing K(+) (60 mmol/L) increased CGRP release as measured by ELISA. Stimulation-evoked CGRP release was attenuated by 30 min pre-treatment with the EETs antagonist 14,15-epoxyeicosa-5(Z)-enoic acid (14,15-EEZE, 10 μmol/L). K(+) stimulation elevated CGRP release 2.9 ± 0.3-fold above control levels, whereas in the presence of 14,15-EEZE K(+)-evoked CGRP release was significantly reduced to 1.1 ± 0.2-fold above control release (p < 0.01 anova, n = 6). 14,15-EEZE likewise attenuated capsaicin-evoked CGRP release from trigeminal ganglion neurons (p < 0.05 anova, n = 6). Similarly, pre-treatment with the cytochrome P450 epoxygenase inhibitor attenuated stimulation-evoked CGRP release. These data demonstrate that EETs are endogenous constituents of rat trigeminal ganglion neurons and suggest that they may act as intracellular regulators of neuropeptide release, which may have important clinical implications for treatment of migraine, stroke and vasospasm after subarachnoid hemorrhage.

Epoxyeicosanoid signaling in CNS function and disease

Epoxyeicosatrienoic acids (EETs) are arachidonic acid metabolites of cytochrome P450 epoxygenase enzymes recognized as key players in vascular function and disease, primarily attributed to their potent vasodilator, anti-inflammatory and pro-angiogenic effects. Although EETs' actions in the central nervous system (CNS) appear to parallel those in peripheral tissue, accumulating evidence suggests that epoxyeicosanoid signaling plays different roles in neural tissue compared to peripheral tissue; roles that reflect distinct CNS functions, cellular makeup and intercellular relationships. This is exhibited at many levels including the expression of EETs-synthetic and -metabolic enzymes in central neurons and glial cells, EETs' role in neuro-glio-vascular coupling during cortical functional activation, the capacity for interaction between epoxyeicosanoid and neuroactive endocannabinoid signaling pathways, and the regulation of neurohormone and neuropeptide release by endogenous EETs. The ability of several CNS cell types to produce and respond to EETs suggests that epoxyeicosanoid signaling is a key integrator of cell-cell communication in the CNS, coordinating cellular responses across different cell types. Under pathophysiological conditions, such as cerebral ischemia, EETs protect neurons, astroglia and vascular endothelium, thus preserving the integrity of cellular networks unique to and essential for proper CNS function. Recognition of EETs' intimate involvement in CNS function in addition to their multi-cellular protective profile has inspired the development of therapeutic strategies against CNS diseases such as cerebral ischemia, tumors, and neural pain and inflammation that are based on targeting the cellular actions of EETs or their biosynthetic and metabolizing enzymes. Based upon the emerging importance of epoxyeicosanoids in cellular function and disease unique to neural systems, we propose that the actions of "neuroactive EETs" are best considered separately, and not in aggregate with all other peripheral EETs functions.

Arachidonic acid metabolites inhibit the stimulatory effect of angiotensin II in renal proximal tubules

Angiotensin II (Ang II) regulates renal proximal transport in a biphasic way via Ang II type 1 receptor (AT1). Whereas extracellular signal-regulated kinase (ERK) activation mediates the stimulatory effect, cytosolic phospholipase A2 (cPLA2) mediates the inhibitory effect independently of ERK. In this study, we tested the hypothesis that the cPLA2/P450 epoxygenase pathway might work to suppress the Ang II-mediated ERK activation. In the presence of arachidonic acid or 5,6-epoxyeicosatrienoic acid (EET), Ang II failed to stimulate the Na-HCO3 cotransporter activity in renal proximal tubules isolated from wild-type, AT1A-deficient, and cPLA2-alpha-deficient mice. In addition, Ang II failed to induce a significant ERK phosphorylation in the presence of arachidonic acid or 5,6-EET. Arachidonic acid or 5,6-EET also suppressed the stimulatory effect of Ang II on net proximal tubule bicarbonate absorption without changing cell Ca2+ concentrations. These results indicate that the cPLA2-alpha/P450/EET pathway blocks the stimulatory effect of Ang II by suppressing the ERK activation. Thus, the cPLA2-alpha/P450/EET pathway may operate as a unique negative feedback mechanism to attenuate excessive Ang II activity in the renal proximal tubules, where extremely high concentrations of Ang II are found.

Increased blood pressure in mice lacking cytochrome P450 2J5

The cytochrome P450 (CYP) enzymes participate in a wide range of biochemical functions, including metabolism of arachidonic acid and steroid hormones. Mouse CYP2J5 is abundant in the kidney where its products, the cis-epoxyeicosatrienoic acids (EETs), modulate sodium transport and vascular tone. To define the physiological role of CYP2J5 in the kidney, knockout mice were generated using a conventional gene targeting approach. Cyp2j5 (-/-) mice develop normally and exhibit no overt renal pathology. While renal EET biosynthesis was apparently unaffected by the absence of CYP2J5, deficiency of this CYP in female mice was associated with increased blood pressure, enhanced proximal tubular transport rates, and exaggerated afferent arteriolar responses to angiotensin II and endothelin I. Interestingly, plasma 17beta-estradiol levels were reduced in female Cyp2j5 (-/-) mice and estrogen replacement restored blood pressure and vascular responsiveness to normal levels. There was no evidence of enhanced estrogen metabolism, or altered expression or activities of steroidogenic enzymes in female Cyp2j5 (-/-) mice, but their plasma levels of luteinizing hormone and follicle stimulating hormone were inappropriately low. Together, our findings illustrate a sex-specific role for CYP2J5 in regulation of blood pressure, proximal tubular transport, and afferent arteriolar responsiveness via an estrogen-dependent mechanism.

Evidence for a membrane site of action for 14,15-EET on expression of aromatase in vascular smooth muscle

Epoxyeicosatrienoic acids (EETs) are synthesized in the endothelial cells of vascular tissues. They are released from the endothelial cells and produce relaxation of the smooth muscle cells by hyperpolarization. The present findings demonstrate that EETs also regulate aromatase activity in vascular smooth muscle cells. Exposure of cultured rat aortic smooth muscle cells to either 1 microM 14,15-EET or 1 microM 11,12-EET inhibits dibutyryl cAMP-induced aromatase activity by 80-100%. 11,12-Dihydroxyeicosatrienoic acid, the hydration product of 11,12-EET, has no effect on dibutyryl cAMP-induced vascular smooth muscle aromatase activity. In contrast to 14,15-EET, the N-methylsulfanilamide derivative of 14,15-EET (14,15-EET-SA) was neither metabolized nor incorporated into cell lipids, but it retained the ability to inhibit cAMP-induced aromatase activity. Furthermore, the 14,15-EET-SA inhibition of cAMP-induced aromatase activity persisted when the sulfanilamide derivative of 14,15-EET was covalently tethered to silica beads (average diameter, 0.5 microm), which restricted 14,15-EET-SA from entering the cell. These data are consistent with the presence of a receptor for EETs in the plasma membrane and support the hypothesis that the inhibition of aromatase by EETs is initiated by the interaction of EET with the putative plasma membrane receptor.

The production of progesterone and 5,6-epoxyeicosatrienoic acid by human granulosa cells

Previous investigations have implicated epoxygenase metabolites of arachidonic acid in the control of steroidogenesis in luteinised granulosa cells. The aim of this study was to assess this hypothesis further. We first determined the responsiveness of the cells in vitro to three different stimuli, namely luteinising hormone (LH), interleukin-1beta (IL-1beta), and dibutyryl cyclic AMP (db. cyclic AMP). Their effects were time-dependent, in that progesterone production from cells incubated for 3 days prior to stimulation responded strongly to db. cyclic AMP, to a lesser extent to LH and not to IL-1beta. After 6 days of preincubation, all three stimuli increased progesterone production, and this preincubation period was used in the remainder of the study.LH and IL-1beta increased the intracellular levels of 5,6-epoxyeicosatrienoic acid (5,6-EpETrE) maximally after 10 min, whereas db. cyclic AMP had a more rapid effect within 2-5 min. There were no changes in levels of 14,15-epoxyeicosatrienoic acid (14,15-EpETrE), indicating that the effect was specific. Levels of dihydroxy derivatives of arachidonic acid were also increased, suggesting rapid metabolism of 5,6-EpETrE to inactive 5,6-DiHETrE. The effects of 5,6-EpETrE on progesterone production were transient, which may be due to the lability of this compound in solution, and limited passage into the granulosa-luteal cell cytoplasm. These results support a role for 5,6-EpETrE in the production of progesterone by human granulosa-luteal cells.

Role of melittin-like region within phospholipase A(2)-activating protein in biological function

Phospholipase A(2)-activating protein (PLAA) has been implicated in the production of prostaglandins (e.g. PGE(2)) via activation of phospholipases in various stimulated cell types. Human PLAA, with 738 amino acid (aa) residues, contains a region of 38% homology (aa 503-538) with the 26-aa long melittin peptide, a major component of bee venom and a reported regulator of phospholipase A(2) and phospholipase D activity. To learn more about the role of PLAA in the production of eicosanoids and other inflammatory mediators, we synthesized a murine PLAA peptide (36-aa long) having homology to melittin, as well as to human and rat PLAA. The PLAA peptide and melittin increased the expression of genes encoding the proinflammatory cytokine tumor necrosis factor alpha (TNFalpha) and cyclooxygenase-2 (COX-2), which is involved in PGE(2) production. We determined that the C-terminal region of the PLAA peptide (aa 515-538) was essential, since truncation of the C-terminal end of the PLAA peptide significantly reduced expression of genes encoding TNFalpha and COX-2 in macrophages. We concluded that PLAA could be important in the regulation of the inflammatory response because of its stimulatory effects on eicosanoid and cytokine synthesis. Consequently, control of plaa gene expression could be a target for the development of new drugs to control the inflammatory response.

Actions of epoxyeicosatrienoic acid on large-conductance Ca(2+)-activated K(+) channels in pituitary GH(3) cells

Epoxyeicosatrienoic acids (EETs), a family of cytochrome P450 epoxygenase metabolites of arachidonic acid, are believed to have an autocrine or paracrine role in the regulation of neurons or neuroendocrine cells. The effects of 14,15-EET on ionic currents were investigated in rat pituitary GH(3) cells. In the whole-cell configuration, 14,15-EET (3 microM) reversibly increased the amplitude of the Ca(2+)-activated K(+) current (I(K(Ca))). The 14, 15-EET-induced increase in I(K(Ca)) was unaffected in the presence of 10 microM thyrotropin-releasing hormone externally or 10 microM inositol trisphosphate in the recording pipette. In cells preincubated with pertussis toxin or herbimycin A, the 14, 15-EET-induced increase in I(K(Ca)) was also not changed. In the inside-out configuration, 14,15-EET applied intracellularly did not change single-channel conductance, but did increase the opening probability of large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels. 14,15-EET (3 microM) shifted the activation curve of BK(Ca) channels to less positive membrane potential by approximately 15 mV. The change in the kinetic behavior of BK(Ca) channels caused by 14,15-EET is explained by a lengthening of open and a shortening of closed times. 14,15-EET increased the activity of BK(Ca) channels in a concentration-dependent manner with an EC(50) value of 1 microM. However, 14,15-EET did not affect the Ca(2+) sensitivity of BK(Ca) channels. The present study indicates that 14,15-EET is an opener of BK(Ca) channels in GH(3) cells and that the stimulatory effect of 14, 15-EET on these channels may, at least in part, contribute to the underlying cellular mechanisms by which EETs affect neuronal or neuroendocrine function.

Epoxyeicosatrienoic acids increase intracellular calcium concentration in vascular smooth muscle cells

Epoxyeicosatrienoic acids (EETs) are cytochrome P450-derived metabolites of arachidonic acid. They are potent endogenous vasodilator compounds produced by vascular cells, and EET-induced vasodilation has been attributed to activation of vascular smooth muscle cell (SMC) K(+) channels. However, in some cells, EETs activate Ca(2+) channels, resulting in Ca(2+) influx and increased intracellular Ca(2+) concentration ([Ca(2+)](i)). We investigated whether EETs also can activate Ca(2+) channels in vascular SMC and whether the resultant Ca(2+) influx can influence vascular tone. The 4 EET regioisomers (1 micromol/L) increased porcine aortic SMC [Ca(2+)](i) by 52% to 81%, whereas arachidonic acid, dihydroxyeicosatrienoic acids, and 15-hydroxyeicosatetraenoic acid (1 micromol/L) produced little effect. The increases in [Ca(2+)](i) produced by 14,15-EET were abolished by removal of extracellular Ca(2+) and by pretreatment with verapamil (10 micromol/L), an inhibitor of voltage-dependent (L-type) Ca(2+) channels. 14,15-EET did not alter Ca(2+) signaling induced by norepinephrine and thapsigargin. When administered to porcine coronary artery rings precontracted with a thromboxane mimetic, 14,15-EET produced relaxation. However, when administered to rings precontracted with acetylcholine or KCl, 14,15-EET produced additional contractions. In rings exposed to 10 mmol/L KCl, a concentration that did not affect resting ring tension, 14,15-EET produced small contractions that were abolished by EGTA (3 mmol/L) or verapamil (10 micromol/L). These observations indicate that 14,15-EET enhances [Ca(2+)](i) influx in vascular SMC through voltage-dependent Ca(2+) channels. This 14,15-EET-induced increase in [Ca(i)(2+)] can produce vasoconstriction and therefore may act to modulate EET-induced vasorelaxation.

Inhibition of cardiac L-type calcium channels by epoxyeicosatrienoic acids

Epoxyeicosatrienoic acids (EETs), products of the cytochrome P-450 monooxygenase metabolism of arachidonic acid, can regulate the activity of ion channels. We examined the effects of EETs on cardiac L-type Ca2+ channels that play important roles in regulating cardiac contractility, controlling heart rate, and mediating slow conduction in normal nodal cells and ischemic myocardium. Our experimental approach was to reconstitute porcine L-type Ca2+ channels into planar lipid bilayers where we could control the aqueous and lipid environments of the channels and the regulatory pathways that change channel properties. We found that 20 to 125 nM EETs inhibited the open probability of reconstituted L-type Ca2+ channels, accelerated the inactivation of the channels, and reduced the unitary current amplitude of open channels. There was no selectivity among different EET regioisomers or stereoisomers. When 11,12-EET was esterified to the sn-2 position of phosphatidylcholine, restricting it to the hydrophobic phase of the planar lipid bilayer, the reconstituted channels were similarly inhibited, suggesting that the EET interacts directly with Ca2+ channels through the lipid phase. The inhibitory effects of EET persisted in the presence of microcystin, an inhibitor of protein phosphatases 1 and 2A, suggesting that dephosphorylation was not the mechanism through which these eicosanoids down-regulate channel activity. This inhibition may be an important protective mechanism in the setting of cardiac ischemia where arachidonic acid levels are dramatically increased and EETs have been shown to manifest preconditioning-like effects.

Activation of microsomal cytochrome P450 mono-oxygenase by Ca2+ store depletion and its contribution to Ca2+ entry in porcine aortic endothelial cells

1. We investigated how microsomal cytochrome P450 mono-oxygenase (Cyp450 MO) is regulated in cultured porcine aortic endothelial cells. The hypothesis that a Cyp450 MO-derived metabolite links Ca2+ store depletion and Ca2+ entry was studied further. 2. Microsomal Cyp450 MO was monitored fluorometrically by dealkylation of 1-ethoxypyrene-3,6,8-tris-(dimethyl-sulphonamide; EPSA) in saponin permeabilized cells or in subcellular compartments. Endothelial Ca2+ signalling was measured by a standard fura-2 technique, membrane potential was determined with the potential-sensitive fluorescence dye, bis-(1,3-dibutylbarbituric acid) pentamethine oxonol (DiBAC4(5)) and tyrosine kinase was quantified by measuring the phosphorylation of a immobilized substrate with a horseradish peroxidase labelled phosphotyrosine specific antibody. 3. Depletion of cellular Ca2+ pools with inositol 1,4,5-trisphosphate (IP3), thapsigargin or cyclopiazonic acid activated microsomal Cyp450 MO. Similar to direct Ca2+ store depletion, chelating of intramicrosomal Ca2+ with oxalate stimulated Cyp450 MO activity, while changing cytosolic free Ca2+ failed to influence Cyp450 MO activity. These data indicate that microsomal Cyp450 MO is activated by depletion of IP3-sensitive stores. 4. Besides the common cytochrome P450 inhibitors, econazole, proadifen and miconazole, thiopentone sodium and methohexitone inhibited Cyp450 MO in a concentration-dependent manner. The physiological substrate of Cyp450 MO, arachidonic acid, inhibited EPSA dealkylation. In contrast to most other cytochrome P450 inhibitors used in this study, thiopentone sodium did not directly interfere with Ca2+ entry pathways, membrane hyperpolarization due to K+ channel activation or tyrosine kinase activity. 5. Inhibition of Cyp450 MO by thiopentone sodium diminished Ca2+/Mn2+ entry to Ca2+ store depletion by 43%, while it did not interfere with intracellular Ca2+ release by IP3 or thapsigargin. 6. Cyp450 MO inhibition with thiopentone sodium diminished autacoid-induced membrane hyperpolarization. 7. Induction of Cyp450 MO with dexamethasone/clofibrate for 72 h yielded increases in thapsigargin-induced Cyp450 MO activity (by 35%), Ca2+/Mn2+ entry (by 105%) and membrane hyperpolarization (by 40%). 8. The Cyp450 MO-derived compounds, 11,12 and 5,6-epoxyeicosatrienoic acids (EETs) yielded membrane hyperpolarization, insensitive to thiopentone sodium. 9. These data demonstrate that endothelial Cyp450 MO is activated by Ca2+ store depletion and Cyp450 MO produced compounds that hyperpolarize endothelial cells. 10. The data presented and our previous findings indicate that Cyp450 MO plays a crucial role in the regulation of store-operated Ca2+ influx. We propose that Cyp450 MO-derived EETs constitute a signal for Ca2+ entry activation and increase the driving force for Ca2+ entry by membrane hyperpolarization in porcine aortic endothelial cells.

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.

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.

Formation and metabolism of 14,15-epoxyeicosatrienoic acid by human reproductive tissues

Human granulosa-luteal cells cultured in the presence of arachidonic acid produced low levels of the epoxygenase metabolite 14,15-epoxy-5,8,11-(Z,Z,Z)-eicosatrienoic acid (14,15-EpETrE) as determined by HPLC analysis and gas chromatography mass spectrometry. When authentic 14,15-[3H]EpETrE was incubated with these cells in the absence of serum it was metabolised initially to the dihydroxy derivative (14,15-dihydroxy-5,8,11-eicosatrienoic acid, 14,15-DiHETrE) and subsequently to a number of more polar metabolites as determined by HPLC. Fetal calf serum protected 14,15-EpETrE from metabolism for at least 2 h. A similar pattern of metabolism was obtained when 14,15-[3H]EpETrE was incubated with a human choriocarcinoma cell line (BeWo). Microsomes from this cell line converted arachidonic acid to a large number of radioactive metabolites including 14,15-DiHETrE and 11,12-DiHETrE although there was no evidence for the parent epoxides. These results extend earlier findings that human reproductive tissues produce epoxygenase metabolites, and demonstrate the rapid metabolism of these compounds by intact cells in the absence of serum.

Cytochrome P450 mono-oxygenase-regulated signalling of Ca2+ entry in human and bovine endothelial cells

1. We tested the hypothesis that agonist-stimulated Ca2+ entry, and thus formation of endothelium-derived nitric oxide (EDNO) in vascular endothelial cells, is related to activation of microsomal P450 mono-oxygenase (P450 MO) and the biosynthesis of 5,6-epoxyeicosatrienoic acid (5,6-EET). 2. Several P450 inhibitors diminished the sustained [Ca2+]i plateau response to agonist or intracellular Ca2+ store depletion with ATPase inhibitors by 31-69% (fura-2 technique). Mn2+ influx stimulated by agonists or ATPase inhibitors was prevented by P450 inhibitors. 3. Histamine- or ATPase inhibitor-stimulated formation of EDNO was strongly attenuated (50-83%) by P450 inhibitors, without any effect on EDNO formation by the Ca2+ ionophore A23187, indicating that decreased EDNO synthesis is due specifically to the inhibition of Ca2+ entry by these compounds. 4. Induction of P450 MO by beta-naphthoflavone potentiated agonist-induced Ca2+ and Mn2+ influx by 60 and 53%, respectively. Intracellular Ca2+ release remained unchanged. 5. The P450 MO product, 5,6-EET (< 156 nmol l-1), activated Ca2+/Mn2+ entry without any depletion of intracellular Ca2+ stores. The 5,6-EET-stimulated Ca2+/Mn2+ entry was not affected by P450 inhibitors. 6. As with the bradykinin-stimulated Ca2+ entry pathway, the 5,6-EET-activated Ca2+ entry pathway was permeable to Mn2+ and Ba2+, sensitive to Ni2+, La3+ and membrane depolarization, and insensitive to the removal of extracellular Na+ or the organic Ca2+ antagonist, nitrendipine. 7. In the presence of 5,6-EET, stimulation with bradykinin only transiently increased [Ca2+]i. Vice versa, 5,6-EET failed to increase [Ca2+]i further in bradykinin-stimulated cells. The sustained [Ca2+]i plateau phase induced by a co-stimulation with bradykinin and 5,6-EET was identical to that observed with bradykinin or 5,6-EET alone. 8. These results demonstrate that Ca2+ entry induced by the P450 MO product, 5,6-EET, is indistinguishable to that observed by stimulation with bradykinin. 9. All data support our hypothesis that depletion of endothelial Ca2+ stores activates microsomal P450 MO which in turn synthesizes 5,6-EET. We propose that the arachidonic acid metabolite 5,6-EET or one of its metabolites is a second messenger for activation of endothelial Ca2+ entry.

Effects of 5,8,11,14-eicosatetraynoic acid on thapsigargin-induced calcium entry, and intracellular pH in thyroid FRTL-5 cells

The effect of 5,8,11,14-eicosatetraynoic acid (ETYA), an inhibitor of lipoxygenase and cytochrome P-450 epoxygenase enzymes, on calcium fluxes was investigated in Fura 2 loaded rat thyroid FRTL-5 cells. ETYA per se released sequestered calcium. ETYA also inhibited calcium influx in thapsigargin-stimulated cells in dose-dependent manner. Addition of calcium to cells treated with ETYA and stimulated with thapsigargin in a calcium-free buffer resulted in a blunted increase in intracellular free calcium compared with the response in control cells. In addition, ETYA per se acidified the cytosol in a dose-dependent manner. Acidification of the cytosol with the K+/H+ ionophore nigericin also decreased thapsigargin-induced calcium entry, but not to the same extent as that seen in cells treated with ETYA. The results suggest that ETYA is a potent modulator of calcium entry, and that part of the inhibitory effect of ETYA may be due to the ETYA-induced acidification of the cytosol.

Thapsigargin-induced calcium entry in FRTL-5 cells: possible dependence on phospholipase A2 activation

Stimulating rat thyroid FRTL-5 cells with agonists that activate the inositol phosphate cascade results in the release of sequestered calcium and influx of extracellular calcium. In addition, phospholipase A2 (PLA2) is activated. Since PLA2 is a calcium-dependent enzyme we wanted to investigate the interrelationships between PLA2 activity and the entry of calcium. Stimulating 3H-arachidonic acid (3H-AA)-labelled cells with thapsigargin resulted in a substantial release of 3H-AA. This release was totally abolished in a calcium-free buffer. Pretreatment of Fura 2 loaded cells with 4-bromophenacyl bromide, an inhibitor of PLA2 activity, decreased the thapsigargin-induced entry of calcium, suggesting a role for PLA2 in the regulation of calcium entry. In cells treated with nordihydroguaiaretic acid (NDGA), clotramizole, or econazole, compounds with lipoxygenase and cytochrome P-450 inhibitory actions, the thapsigargin-induced entry of calcium was decreased in a dose-dependent manner. However, treatment of the cells with indomethacin, a cyclooxygenase inhibitor, had no effect on the thapsigargin-induced calcium entry. We also showed that stimulation of the cells with arachidonic acid released sequestered calcium, apparently from the same intracellular pool as did thapsigargin. The results suggested that the calcium-induced PLA2 activation and the metabolism of the produced arachidonic acid by a noncyclooxygenase pathway may be of importance in maintaining calcium entry after releasing sequestered Ca2+ in FRTL-5 cells.

TRH-evoked entry of extracellular calcium in GH4C1 cells: possible importance of arachidonic acid metabolites

Previous studies have shown that stimulating pituitary GH4C1 cells with thyrotropin-releasing hormone (TRH) evoked a biphasic change in cytosolic free Ca2+ concentration ([Ca2+]i): a rapid release of sequestered Ca2+ due to the production of inositol-1,4,5-trisphosphate, and Ca2+ entry via both voltage-operated Ca2+ channels and a presently unknown voltage-independent influx pathway. The aim of the present study was to further evaluate to which extent the TRH-evoked changes in [Ca2+]i were dependent on entry of extracellular Ca2+, and which mechanisms participated in regulating this Ca2+ entry. Pretreatment of the cells with 4-bromophenylacylbromide (an inhibitor of phospholipase A2), nordihydroguaiaretic acid (an inhibitor of lipoxygenase), and econazole (an inhibitor of both lipoxygenase and cytochrome P-450 enzymes), attenuated the TRH-evoked increase in [Ca2+]i, suggesting that noncyclooxygenase metabolites of arachidonic acid or cytochrome P-450 metabolites may participate in regulating the TRH-evoked entry of extracellular Ca2+. Both nordihydroguaiaretic acid and econazole showed a similar inhibition of the Ca2+ entry, as did SKF 96365, a compound previously shown to inhibit receptor-activated Ca2+ entry. We also showed that arachidonic acid per se increased [Ca2+]i, and acidified the cytosol in GH4C1 cells in a dose-dependent manner. The effects of arachidonic acid was reversed by addition of BSA to the cell suspension. The calcium entry and the activation of the metabolism of arachidonic acid may thus be important components of the TRH-evoked signal-transduction pathway in GH4C1 cells.

Oxygenation of unsaturated fatty acids by hepatic microsomes of musk shrew (Suncus murinus)

The cytochrome P-450-dependent monooxygenase in musk shrew (suncus; Suncus murinus) liver microsomes metabolized unsaturated fatty acids (oleic, linoleic, alpha-linolenic and arachidonic acids) to a variety of oxygenated products. alpha-Linolenic acid was the most active substrate. The oxygenation activity increased with an increase in the number of cis double-bonds in the C18 fatty acids. This suggests that the introduction of cis double-bonds in C18 fatty acids is important for the binding of cytochrome P-450 in suncus liver microsomes. Regioselectivity of arachidonic acid oxygenation was observed in suncus liver microsomes; rat liver microsomal cytochrome P-450 generated epoxyeicosatrienoic acids and dihydroxyeicosatrienoic acid as major products while the cytochrome P-450-dependent monooxygenase in suncus liver microsomes yielded omega- and (omega-1)-hydroxyarachidonic acids as major reaction products.

Importance of arachidonic acid metabolites in regulating ATP-induced calcium fluxes in thyroid FRTL-5 cells

Stimulating rat thyroid FRTL-5 cells with the purinergic agonist ATP activates both the inositol phosphate signal-transduction pathway and the phospholipase A2 pathway. In the present study we wanted to investigate the possible inter-relationships between these two systems during ATP-induced changes in intracellular free calcium ([Ca2+]i). Pretreatment of Fura-2 loaded cells with 4-bromophenylacyl, an inhibitor of phospholipase A2, had no effect on the ATP-induced entry of Ca2+ but inhibited the release of sequestered Ca2+. Nordihydroguaiaretic acid (NDGA), a lipoxygenase inhibitor, and 5,8,11,14-eicosatetraynoic acid (ETYA), an inhibitor of cytochrome P-450 enzymes, attenuated the ATP-evoked transient increase in [Ca2+]i. Furthermore, the capacitative entry of Ca2+ was also attenuated in NDGA- and ETYA-treated cells stimulated with ATP. Similar results were obtained using econazole, an inhibitor of cytochrome P-450 enzymes. However, treatment of the cells with indomethacin, a cyclooxygenase inhibitor, had no effect on the ATP-evoked response in [Ca2+]i. We also showed that stimulation of intact or permeabilized FRTL-5 cells with arachidonic acid released sequestered calcium. This calcium originated, at least in part, from an IP3 sensitive calcium pool. In addition, arachidonic acid rapidly acidified the cytosol. The results suggest that metabolism of arachidonic acid by a non-cyclooxygenase pathway is of importance in supporting agonist-induced calcium fluxes evoked via stimulation of the inositol phosphate pathway in FRTL-5 cells. Furthermore, arachidonic acid per se may modify agonist-induced calcium fluxes in these cells.

Oxygenation of polyunsaturated fatty acids by cytochrome P450 monooxygenases

Polyunsaturated fatty acids can be oxygenated by P450 in different ways--by epoxidation, by hydroxylation of the omega-side chain, by allylic and bis-allylic hydroxylation and by hydroxylation with double bond migration. Major organs for these oxygenations are the liver and the kidney. P450 is an ubiquitous enzyme. It is therefore not surprising that some of these reactions have been found in other organs and tissues. Many observations indicate that P450 oxygenates arachidonic acid in vivo in man and in experimental animals. This is hardly surprising. omega-Oxidation was discovered in vivo 60 years ago. It was more unexpected that biological activities have been associated with many of the P450 metabolites of arachidonic acid, at least in pharmacological doses. Epoxygenase metabolites of arachidonic acid have attracted the largest interest. In their critical review on epoxygenase metabolism of arachidonic acid in 1989, Fitzpatrick and Murphy pointed out some major differences between the PGH synthase, the lipoxygenase and the P450 pathways of arachidonic acid metabolism. Their main points are still valid and have only to be modified slightly in the light of recent results. First, lipoxygenases show a marked regiospecificity and stereospecificity, while many P450 seem to lack this specificity. There are, however, P450 isozymes which catalyse stereospecific epoxidations or hydroxylations. Many hydroxylases and at least some epoxygenases also show regiospecificity, i.e. oxygenate only one double bond or one specific carbon of the fatty acid substrate. In addition, preference for arachidonic acid and eicosapentaenoic acid may occur in the sense that other fatty acids are oxygenated with less regiospecificity. A more important difference is that prostaglandins and leukotrienes affect specific and well characterised receptors in cell membranes, while receptors for epoxides of arachidonic acid or other P450 metabolites have not been characterised. Nevertheless, epoxides of arachidonic acid have been found to induce a large number of different pharmacological effects. In some systems, effects have been noted at pm concentrations which might conceivably be in the physiological concentration range of these epoxides, e.g. after release from phospholipids by phospholipase A2. An intriguing possibility is that the effects of [Ca]i on different ion channels might possibly explain their biological actions. In situations when pharmacological doses are used, metabolism to epoxyprostanoids or other interactions with PGH synthase could also be of importance. Finally, one report on a specific receptor for 14R,15S-EpETrE in mononuclear cell membranes has just been published.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolism of arachidonic acid to epoxyeicosatrienoic acids, hydroxyeicosatetraenoic acids, and prostaglandins in cultured rat hippocampal astrocytes

We have recently shown that brain slices are capable of metabolizing arachidonic acid by the epoxygenase pathway. The purpose of this study was to begin to determine the ability of individual brain cell types to form epoxygenase metabolites. We have examined the astrocyte epoxygenase pathway and have also confirmed metabolism by the cyclooxygenase and lipoxygenase enzyme systems. Cultured rat hippocampal astrocyte homogenate, when incubated with radiolabeled [3H]arachidonic acid, formed products that eluted in four major groups designated as R17-30, R42-50, R51-82, and R83-90 based on their retention times in reverse-phase HPLC. These fractions were further segregated into as many as 13 peaks by normal-phase HPLC and a second reverse-phase HPLC system. The principal components in each peak were structurally characterized by gas chromatography/electron impact-mass spectrometry. Based on HPLC retention times and gas chromatography/electron impact-mass spectrometry analysis, the more polar fractions (R17-30) contained prostaglandin D2 as the major cyclooxygenase product. Minor products included 6-keto prostaglandin F1 alpha, prostaglandin E2, prostaglandin F2 alpha, and thromboxane B2. Fractions R42-50, R51-82, and R83-90 contained epoxygenase and lipoxygenase-like products. The major metabolite in fractions R83-90 was 5,6-epoxyeicosatrienoic acid (EET). Fractions R51-82 contained 14,15- and 8,9-EETs, 12- and 5-hydroxyeicosatetraenoic acids, and 8,9- and 5,6-dihydroxyeicosatrienoic acids (DHETs). In fractions R42-50, 14,15-DHET was the major product. When radiolabeled [3H]14,15-EET was incubated with astrocyte homogenate, it was rapidly metabolized to [3H]14,15-DHET. The metabolism was inhibited by submicromolar concentration of 4-phenylchalcone oxide, a potent inhibitor of epoxide hydrolase activity. Formation of other polar metabolites such as triols or epoxy alcohols from 14,15-DHET was not observed. In conclusion, astrocytes readily metabolize arachidonic acid to 14,15-EET, 5,6-EET, and their vicinal-diols. Previous studies suggest these products may affect neuronal function and cerebral blood flow.

Complex effects of arachidonic acid and its lipoxygenase products on cytosolic calcium in GH3 cells

The mechanisms by which arachidonic acid (AA) and its metabolites stimulate prolactin release from pituitary cells are not understood. Because Ca2+ ions are pivotal to exocytosis, we investigated the effects of AA metabolites on intracellular calcium concentration ([Ca2+]i) and membrane ionic currents using dual-excitation microspectrofluorimetry and whole cell patch-clamp techniques in GH3/B6 pituitary line cells. AA (1 microM) had a biphasic effect on [Ca2+]i, mobilization of intracellular Ca2+, followed by stimulation of Ca2+ entry. Only the latter appeared to result from the degradation of AA through the lipoxygenase pathway. Indomethacin (Indo, 10 microM) and lipoxygenase products 5-, 12-, and 15-hydroxyeicosatetraenoic acid (HETE, 1 microM), increased action potential duration (12-HETE) or frequency (5- and 15-HETE, Indo). These effects depended on inhibition of d-tubocurarine- or tetraethylammonium-sensitive K+ conductances and stimulation of voltage-dependent Ca2+ channels. Refilling of intracellular Ca2+ stores, and Ca2+ efflux, may also be stimulated. Our results demonstrate a control of [Ca2+]i by a second messenger (AA) and its metabolites (HETEs).

Brain synthesis and cerebrovascular action of epoxygenase metabolites of arachidonic acid

The purpose of this study was to determine if whole brain makes epoxygenase metabolites of arachidonic acid and, if so, whether they are vasoactive on the cerebral microcirculation. Blood-free mouse brain slices were incubated with exogenous radiolabeled arachidonic acid, and the extracted metabolites were resolved by HPLC. Metabolite structures were confirmed by gas chromatography/mass spectrometry. In addition to prostaglandins, leukotriene B4, and hydroxyeicosatetraenoic acids, mouse brain metabolized arachidonic acid into several other compounds. Among them, we identified 5,6- and 14,15-epoxyeicosatrienoic acid. Next, we tested the effect of topical application of brain-synthesized 5,6-epoxyeicosatrienoic acid and synthetic epoxyeicosatrienoic acids on in vivo rabbit cerebral arteriolar diameter using the cranial window technique and in vivo microscopy. Brain-synthesized 5,6-epoxyeicosatrienoic acid caused a transient 28% arteriolar dilation, similar to that produced by 5 micrograms/ml of synthetic 5,6-epoxyeicosatrienoic acid. A concentration of synthetic 14,15- and 11,12-epoxyeicosatrienoic acid of 5 micrograms/ml CSF had little or no effect on diameter, whereas 8,9-epoxyeicosatrienoic acid caused a maximum dilation of 8%. These studies suggest that brain-synthesized 5,6-epoxyeicosatrienoic acid may play a role in the normal or pathophysiological regulation of the cerebral microcirculation.

An epoxygenase metabolite of arachidonic acid mediates angiotensin II-induced rises in cytosolic calcium in rabbit proximal tubule epithelial cells

Previous studies from this and other laboratories have shown that angiotensin II (AII) induces [Ca2+]i transients in proximal tubular epithelium independent of phospholipase C. AII also stimulates formation of 5,6-epoxyeicosatrienoic acid (5,6-EET) from arachidonic acid by a cytochrome P450 epoxygenase and decreases Na+ transport in the same concentration range. Because 5,6-EET mimics AII with regard to Na+ transport, it effects on calcium mobilization were evaluated. [Ca2+]i was measured by video microscopy with the fluorescent indicator fura-2 employing cultured rabbit proximal tubule. AII-induced [Ca2+]i transients were enhanced by arachidonic acid and attenuated by ketoconazole, an inhibitor of cytochrome P450 epoxygenases. Arachidonic acid also elicited a [Ca2+]i transient that was attenuated by ketoconazole. 5,6-EET augmented [Ca2+]i similar to that seen with AII, but was unaffected by ketoconazole. By contrast, the other regioisomers (8,9-, 11,12-, and 14,15-EET) were much less potent. [Ca2+]i transients resulted from influx through verapamil- and nifedipine-sensitive channels. These results suggest a novel mechanism for AII-induced Ca mobilization in proximal tubule involving cytochrome P450-dependent arachidonic acid metabolism and Ca influx through voltage-sensitive channels.

Formation of epoxyeicosatrienoic acids from arachidonic acid by cultured rat aortic smooth muscle cell microsomes

The vasodilatory effect of epoxyeicosatrienoic acids (EpETrE), especially 5(6)-EpETrE, has been reported recently and a role of P-450-dependent arachidonic acid monooxygenase metabolites was suggested in vasoregulation. Accordingly, the presence of P-450-dependent arachidonic acid monooxygenase was investigated in rat aortic smooth muscle cells. Incubation of the microsomes of rat cultured aortic smooth muscle cells with 14C-arachidonic acid in the presence of 1 mM NADPH resulted in the formation of oxygenated metabolites. The metabolites were separated and purified by reverse phase and straight phase high performance liquid chromatography and identified by gas chromatography-mass spectrometry. Identified metabolites were 5(6)-EpETrE, 5,6-dihydroxyeicosatrienoic acid (DiHETrE), and 14,15-DiHETrE. The formation of these metabolites was totally dependent on the presence of NADPH, and inhibitors of cytochrome P-450-dependent enzymes, SKF-525A and metyrapone, reduced the formation of these metabolites. This is the first report that cytochrome P-450-dependent arachidonic acid metabolites, especially 5(6)-EpETrE and 14(15)-EpETrE, can be produced in the microsomes of vascular smooth muscle cells of rats.

Possible involvement of arachidonic acid metabolites of cytochrome P450 monooxygenase pathway in vasopressin-stimulated glycogenolysis in isolated rat hepatocytes

Arachidonic acid (AA) is reported to be metabolized by three major pathways, i.e., cyclooxygenase (CO), lipoxygenase (LO), and NADPH-dependent cytochrome P450 monooxygenase (MO) pathways. Monooxygenase metabolites of AA have been proposed to play an important role in hormone action in various cells. Recently it was reported that the MO pathway may exist in rat liver. The present study was carried out to investigate the role of MO metabolites in vasopressin-induced glycogenolysis in isolated rat hepatocytes. The pretreatment of isolated rat hepatocytes with eicosatetraynoic acid (ETYA), an inhibitor of CO, LO, and MO pathways, and ketoconazole and SKF 525A, inhibitors of the MO pathway, dose-dependently reduced vasopressin-induced phosphorylase activation, while the pretreatment with indomethacin, an inhibitor of the CO pathway, had no effect. The increment of cytosolic calcium concentration in vasopressin-stimulated hepatocytes was also dose-dependently decreased by ETYA, ketoconazole, and SKF 525A. In vitro addition of epoxyeicosatrienoic acid (EET) dose-dependently increased both phosphorylase a activity and cytosolic calcium concentration. 14,15-EET was the most potent among four regioisomeric EETs. These results suggest that MO metabolites of AA, most likely EETs, may be involved in vasopressin-induced glycogenolysis probably via the activation of phosphorylase by increasing the cytosolic calcium concentration.

Arachidonic acid metabolites by cytochrome P-450 dependent monooxygenase pathway in bovine adrenal fasciculata cells

[1-14C]Arachidonic acid was incubated with microsomes of bovine adrenal fasciculata cells in the presence of 1 mM NADPH for 30 min at 37 degrees C. The metabolites were separated and purified by reverse phase high performance liquid chromatography, and identified by gas chromatography-mass spectrometry. Identified metabolites were four dihydroxyeicosatrienoic acids (DHTs) (5,6-, 8,9-, 11,12-, 14,15-DHTs), 20-hydroxyeicosatetraenoic acid and eicosatetradioic acid. The formation of these metabolites was dependent on NADPH and inhibited by SKF-525A. 14,15-DHT was also formed by isolated bovine adrenal fasciculata cells. These results indicate that cytochrome P-450 dependent arachidonate monooxygenase pathway may exist in bovine adrenal fasciculata cells. Addition of the chemically synthesized epoxyeicosatrienoic acids (EETs) to isolated bovine adrenal fasciculata cells stimulated cortisol production. Among four regioisomeric EETs, 14,15-EET was most potent and stimulated steroidogenesis in a dose-related manner over a range of 0.5 to 5.0 microM.

5,6-Epoxyeicosatrienoic acid stimulates growth hormone release in rat anterior pituitary cells

The effect of arachidonic acid and some of its metabolites have been examined in rat anterior pituitary cells for their ability to release growth hormone. The cytochrome P-450 metabolite, 5,6-epoxyeicosatrienoic acid is a much more effective growth-hormone releasing agent than 15-hydroxyeicosatetraenoic acid, 15-hydroxyeicosatetraenoic acid methyl ester, 5-hydroxyeicosatetraenoic acid or arachidonic acid. The release of growth hormone is rapid, dose-dependent and reaches an apparent saturation after eight minutes. These studies described herein provide evidence that lipoxygenase and cyclooxygenase products of arachidonic acid are less potent while cytochrome P-450 products are more potent in the release of growth hormone from anterior pituitary cells.

A novel pool of rat liver inositol and ethanolamine phospholipids contains epoxyeicosatrienoic acids (EETs)

A pool of phosphatidylinositol and phosphatidylethanolamine containing epoxyeicosatrienoic acids esterified to the sn-2 position of the glycerol moiety has been identified in rat liver. The mole fraction of epoxide containing phosphatidylinositol is 7-8 times that of phosphatidylethanolamine.

Arachidonic acid epoxygenase: detection of epoxyeicosatrienoic acids in human urine

Epoxyeicosatrienoic acids, metabolites of the cytochrome P-450-mediated epoxygenase reaction, were detected in human urine by gas chromatographic-mass spectroscopic techniques after conversion to their hydrogenated and non-hydrogenated methyl and pentafluorobenzyl esters. Initial analysis of the regioisomeric composition utilizing the corresponding hydrogenated pentafluorobenzyl esters revealed the presence of the 8,9- and 14,15-isomers.