Regulation of calcium influx and catecholamine secretion in chromaffin cells by a cytochrome P450 metabolite of arachidonic acid

Two Pools of Epoxyeicosatrienoic Acids in Humans: Alterations in Salt-Sensitive Normotensive Subjects

We measured epoxyeicosatrienoic acids (EETs) and dihydroxyeicosatrienoic acids (DHETs) in 21 normotensive subjects classified as salt resistant (13) or salt sensitive (8) with an inpatient protocol of salt loading (460 mEq Na⁺/24 hours, HiNa) and depletion (10 mEq Na⁺/24 hours+furosemide 40 mg×3, LoNa). No urine EETs were detected; hence, enzyme linked innumosorbent assay 14,15-DHETs (dihydroxyeicosatrienoic acids) were considered the total converted 14,15-urine pool. We report ultra-performance liquid chromatography/tandem mass spectrometry plasma EETs, DHETs, and their sum (plasma total pool) for the 3 regioisomers (8,9-, 11,12-, 14,15-) and their sum (08,15-). In salt-resistant subjects, urine total pool was unchanged by HiNa, decreased by LoNa, and correlated with urine sodium excretion, fractional excretion of Na⁺, and Na⁺/K⁺ ratio for the 3 days of the experiment combined (P<0.03). In contrast, plasma total pool increased in LoNa and did not correlate with natriuresis or Na⁺/K⁺ ratio but showed correlations between EETs, blood pressures, and catecholamines and between DHETs and aldosterone (P<0.03). Urine total pool of salt-sensitive was lower than that of salt-resistant subjects in certain phases of the experiment, lacked responses to changes in salt balance, and exhibited limited correlations with natriuresis and Na⁺/K⁺ ratio during LoNa only. Plasma total pool of salt-sensitive was lower than in salt-resistant subjects and did not correlate with blood pressures or aldosterone but did with catecholamines. We conclude that the urine total pool reflects a renal pool involved in regulation of natriuresis, whereas the plasma total pools are of systemic origin, uninvolved in Na⁺ excretion, perhaps contributing to regulation of vascular tone. Data suggest that abnormalities in EETs in salt-sensitive subjects participate in their renal or vascular dysfunction, which has potential therapeutic implications.

Off-target effects of MEK inhibitors

The mitogen-activated protein kinases (MAPKs) ERK1/2 regulate numerous cellular processes, including gene transcription, proliferation, and differentiation. The only known substrates of the MAP2Ks MEK1/2 are ERK1/2; thus, MEK inhibitors PD98059, U0126, and PD0325901 have been important tools in determining the functions of ERK1/2. By using these inhibitors and genetically manipulating MEK, we found that ERK1/2 activation is neither sufficient nor necessary for regulated secretion of insulin from pancreatic β cells or secretion of epinephrine from chromaffin cells. We show that both PD98059 and U0126 reduce agonist-induced entry of calcium into cells in a manner independent of their ability to inhibit ERK1/2. Caution should be used when interpreting results from experiments using these compounds.

Proteomics of neuroendocrine secretory vesicles reveal distinct functional systems for biosynthesis and exocytosis of peptide hormones and neurotransmitters

Regulated secretory vesicles produce, store, and secrete active peptide hormones and neurotransmitters that function in cell-cell communication. To gain knowledge of the protein systems involved in such secretory vesicle functions, we analyzed proteins in the soluble and membrane fractions of dense core secretory vesicles purified from neuroendocrine chromaffin cells. Soluble and membrane fractions of these vesicles were subjected to SDS-PAGE separation, and proteins from systematically sectioned gel lanes were identified by microcapillary LC-MS/MS (microLC-MS/MS) of tryptic peptides. The identified proteins revealed functional categories of prohormones, proteases, catecholamine neurotransmitter metabolism, protein folding, redox regulation, ATPases, calcium regulation, signaling components, exocytotic mechanisms, and related functions. Several novel secretory vesicle components involved in proteolysis were identified consisting of cathepsin B, cathepsin D, cystatin C, ubiquitin, and TIMP, as well carboxypeptidase E/H and proprotein convertases that are known to participate in prohormone processing. Significantly, the membrane fraction exclusively contained an extensive number of GTP nucleotide-binding proteins related to Rab, Rho, and Ras signaling molecules, together with SNARE-related proteins and annexins that are involved in trafficking and exocytosis of secretory vesicle components. Membranes also preferentially contained ATPases that regulate proton translocation. These results implicate membrane-specific functions for signaling and exocytosis that allow these secretory vesicles to produce, store, and secrete active peptide hormones and neurotransmitters released from adrenal medulla for the control of physiological functions in health and disease. In summary, this proteomic study illustrates secretory vesicle protein systems utilized for the production and secretion of regulatory factors that control neuroendocrine functions.

Direct coupling between arachidonic acid-induced Ca2+ release and Ca2+ entry in HEK293 cells

Arachidonic acid (AA) modulates intracellular Ca2+ signaling via Ca2+ release or/and Ca2+ entry. However, the mechanism underlies either process is unknown; nor is it clear as to whether the two processes are mechanistically linked. By using Fura2/AM, we found that AA induced mobilization of internal Ca2+ store and an increment in Ca2+, Mn2+ and Ba2+ influx in HEK293 cells. The AA-mediated Ca2+ signaling was not due to AA metabolites, and insensitive to capacitative Ca2+ entry inhibitors. Interestingly, isotetrandrine and Gd3+ inhibited both AA-induced Ca2+ release and Ca2+ entry in a concentration-dependent manner without affecting Ca2+ discharge caused by carbachol, caffeine, or thapsigargin. Additionally, similar pattern of inhibition was observed with tetracaine treatment. More importantly, the three compounds exhibited almost equal potent inhibition of AA-initiated Ca2+ release as well as Ca2+ influx. Therefore, this study, for the first time, provides evidence for a direct coupling between AA-mediated Ca2+ release and Ca2+ entry.

Stable 5,6-epoxyeicosatrienoic acid analog relaxes coronary arteries through potassium channel activation

5,6-epoxyeicosatrienoic acid (5,6-EET) is a cytochrome P450 epoxygenase metabolite of arachidonic acid that causes vasorelaxation. However, investigations of its role in biological systems have been limited by its chemical instability. We developed a stable agonist of 5,6-EET, 5-(pentadeca-3(Z),6(Z),9(Z)-trienyloxy)pentanoic acid (PTPA), in which the 5,6-epoxide was replaced with a 5-ether. PTPA obviates chemical and enzymatic hydrolysis. In bovine coronary artery rings precontracted with U46619, PTPA (1 nmol/L to 10 micromol/L) induced concentration-dependent relaxations, with maximal relaxation of 86+/-5% and EC50 of 1 micromol/L. The relaxations were inhibited by the cyclooxygenase inhibitor indomethacin (10 micromol/L; max relaxation 43+/-9%); the ATP-sensitive K+ channel inhibitor glybenclamide (10 micromol/L; max relaxation 49+/-6%); and the large conductance calcium-activated K+ channel inhibitor iberiotoxin (100 nmol/L; max relaxation 38+/-6%) and abolished by the combination of iberiotoxin with indomethacin or glybenclamide or increasing extracellular K+ to 20 mmol/L. Whole-cell outward K+ current was increased nearly 6-fold by PTPA (10 micromol/L), which was also blocked by iberiotoxin. Additionally, we synthesized 5-(pentadeca-6(Z),9(Z)-dienyloxy)pentanoic acid and 5-(pentadeca-3(Z),9(Z)-dienyloxy)pentanoic acid (PDPA), PTPA analogs that lack the 8,9 or 11,12 double bonds of arachidonic acid and therefore are not substrates for cyclooxygenase. The PDPAs caused concentration-dependent relaxations (max relaxations 46+/-13% and 52+/-7%, respectively; EC50 1micromol/L), which were not altered by glybenclamide but blocked by iberiotoxin. These studies suggested that PTPA induces relaxation through 2 mechanisms: (1) cyclooxygenase-dependent metabolism to 5-ether-containing prostaglandins that activate ATP-sensitive K+ channels and (2) activation of smooth muscle large conductance calcium-activated K+ channels. PDPAs only activate large conductance calcium-activated K+ channels.

Mechanisms in histamine-mediated secretion from adrenal chromaffin cells

The great majority of the sustained secretory response of adrenal chromaffin cells to histamine is due to extracellular Ca(2+) influx through voltage-operated Ca(2+) channels (VOCCs). This is likely to be true also for other G protein-coupled receptor (GPCR) agonists that evoke catecholamine secretion from these cells. However, the mechanism by which these GPCRs activate VOCCs is not yet clear. A substantial amount of data have established that histamine acts on H(1) receptors to activate phospholipase C via a Pertussis toxin-resistant G protein, causing the production of inositol 1,4,5-trisphosphate and the mobilisation of store Ca(2+); however, the molecular events that lead to the activation of the VOCCs remain undefined. This review will summarise the known actions of histamine on cellular signalling pathways in adrenal chromaffin cells and relate them to the activation of extracellular Ca(2+) influx through voltage-operated channels, which evokes catecholamine secretion. These actions provide insight into how other GPCRs might activate Ca(2+) influx in many excitable and non-excitable cells.

Effects of epoxyeicosatrienoic acids on polymorphonuclear leukocyte function

During periods of ischemia and vascular injury, factors are released which recruit monocytes and polymorphonuclear leukocytes (PMNs) to the site of injury by promoting adherence to the endothelium and transmigration across the endothelial cell (EC) layer. During coronary artery stenosis, we have shown that the endothelium-derived, cytochrome P450 metabolites of arachidonic acid, the epoxyeicosatrienoic acids (EETs), are elevated. Therefore, we examined if the EETs could stimulate PMN adherence to cultured ECs. Pretreatment of ECs with EETs for either 30 min or 4 hr did not alter the adherence of 51Cr-labelled PMNs to ECs while phorbol myristate acetate (PMA) produced a 4-fold increase in PMN adherence. The combination of EETs and PMA did not significantly augment or diminish PMA-induced PMN adherence to ECs. When ECs and 51Cr-labelled PMNs were coincubated, treatment with EETs alone did not alter PMN adherence. However, when EETs and PMA were added together during the coincubation of ECs and 51Cr-labelled PMNs, the EETs produced a concentration-related decrease in PMN adherence. Microscopic analysis of the culture media bathing the cells revealed aggregates of the labeled PMNs. We examined the effects of the EETs on PMN aggregation. 8,9-EET (10, 50, and 100 microM) increased PMN aggregation (7 +/- 3, 35 +/- 10, and 65 +/- 11%) and intracellular calcium by 1.7 +/- 0.5, 4.7 +/- 1.4, and 6.8 +/- 2.3-fold above basal. 5,6-, 11,2- and 14,15-EETs also stimulated aggregation. FMLP stimulated the production of superoxide; however, 8,9-EET did not. These observations indicate that the decrease in PMN adherence observed in the coincubation experiment is the result of EET-induced PMN aggregation. Given the increase in EET production during coronary artery stenosis, these data may provide insight into their potential biological significance during myocardial ischemia and vascular injury.

Liquid chromatographic-electrospray ionization-mass spectrometric analysis of cytochrome P450 metabolites of arachidonic acid

Arachidonic acid (AA) can be metabolized by cytochrome P450 (CYP) enzymes to many biologically active compounds including 5,6-, 8,9-, 11,12-, and 14,15-epoxyeicosatrienoic acids (EETs), their corresponding dihydroxyeicosatrienoic acids (DHETs), and 20-hydroxyeicosatetraenoic acid (20-HETE). These eicosanoids are potent regulators of vascular tone. We developed a liquid chromatography-electrospray ionization-mass spectrometry method to simultaneously determine 5,6-, 8,9-, 11,12-, and 14,15-EETs; 5,6-, 8,9-, 11,12-, and 14,15-DHETs; and 20-HETE. [2H8]EETs, [2H8]DHETs, and [2H2]20-HETE were used as internal standards. These compounds are readily separated on a C18 reverse-phase column using water:acetonitrile with 0.005% acetic acid as a mobile phase. The internal standards, [2H8]EETs, [2H8]DHETs, and [2H2]20-HETE, eluted slightly faster than the natural eicosanoids. The samples were ionized by electrospray with fragmentor voltage of 120 V and detected in a negative mode. The negative ion detection gave a lower background than the positive ion detection for these compounds. These eicosanoids exhibited high abundance of the ions corresponding to [M - 1]-. The m/z = 319, 337, and 319 ions were used for quantitation of EETs, DHETs, and 20-HETE, respectively. The detection limits using selected ion monitoring of these compounds are about 1 pg per injection. The position of functional groups and water content of mobile phase had a significant effect on the sensitivity of detection. Water content of 40% was found to give maximal sensitivity. The method was used to determine EETs, DHETs, and 20-HETE in bovine coronary artery endothelial cells, dog plasma, rat astrocytes, and rat kidney microsome samples.

Determination of EETs using microbore liquid chromatography with fluorescence detection

Epoxyeicosatrienoic acids (EETs) are cytochrome P-450 metabolites of arachidonic acid involved in the regulation of vascular tone. The method of microbore column high-performance liquid chromatography with fluorescence detection was developed to determine 14,15-EET, 11, 12-EET, and the mixture of 8,9-EET and 5,6-EET. Tridecanoic acid (TA) was used as an internal standard. EETs were reacted with 2-(2, 3-naphthalimino)ethyl trifluoromethanesulfonate (NT) to form highly fluorescent derivatives. A C(18) microbore column and a water-acetonitrile mobile phase were used for separation. Samples were excited at 259 nm, and the fluorescence was detected at 395 nm. The overall recoveries were 88% for EETs and 40% for TA. EETs were detected in concentrations as low as 2 pg (signal-to-noise ratio = 3). The method was used to determine the EET production from endothelial cells (ECs). Bradykinin and methacholine (10(-6) M) stimulated an increase in the production of EETs by ECs two- and fivefold, respectively. This sensitive method may be used for determination of EETs at low concentrations normally detected in complex biological samples.

Cytochrome P450 metabolites of arachidonic acid in human placenta

Little is known about the epoxygenase pathway of the arachidonic acid cascade in uterine tissues. In this paper, we describe the formation of epoxyeicosatrienoic acids (EETs) and dihydroxyeicosatrienoic acids (DHETs) in human term placenta after uncomplicated pregnancies. Metabolism of [3H]-arachidonic acid was analyzed in short term tissue cultures of placenta by reverse phase HPLC. Major metabolites coeluted with authentic EETs and DHETs. The formation of EETs was inhibited by carbon monoxide. In non-radioactive incubations with biopsies from seven different placentas, sufficient material for GC/MS analysis was obtained. The combined media were purified by solid phase extraction and reverse phase HPLC. The fraction coeluting with DHETs was derivatized with pentafluorobenzylbromide (PFB) and bis-(trimethylsilyl)-trifluoroacetylacetamide (BSTFA) and analyzed by GC/NICI/MS/MS. 11, 12-DHET and 14, 15-DHET were identified by their mass spectra displaying specific fragments at m/z 149 and m/z 189, respectively. Our results suggest that the epoxygenase pathway is active in human term placenta.