Hepoxilin A3 is the endogenous lipid mediator opposing hypotonic swelling of intact human platelets

Biotransformation of polyunsaturated fatty acids to bioactive hepoxilins and trioxilins by microbial enzymes

Hepoxilins (HXs) and trioxilins (TrXs) are involved in physiological processes such as inflammation, insulin secretion and pain perception in human. They are metabolites of polyunsaturated fatty acids (PUFAs), including arachidonic acid, eicosapentaenoic acid and docosahexaenoic acid, formed by 12-lipoxygenase (LOX) and epoxide hydrolase (EH) expressed by mammalian cells. Here, we identify ten types of HXs and TrXs, produced by the prokaryote Myxococcus xanthus, of which six types are new, namely, HXB₅, HXD₃, HXE₃, TrXB₅, TrXD₃ and TrXE₃. We succeed in the biotransformation of PUFAs into eight types of HXs (>35% conversion) and TrXs (>10% conversion) by expressing M. xanthus 12-LOX or 11-LOX with or without EH in Escherichia coli. We determine 11-hydroxy-eicosatetraenoic acid, HXB₃, HXB₄, HXD₃, TrXB₃ and TrXD₃ as potential peroxisome proliferator-activated receptor-γ partial agonists. These findings may facilitate physiological studies and drug development based on lipid mediators.

Hepoxilins (HXs) and trioxilins (TrXs) are lipid metabolites with roles in inflammation and insulin secretion. Here, the authors discover a prokaryotic source of HXs and TrXs, identify the biosynthetic enzymes and heterologously express HXs and TrXs in E. coli.

12-lipoxygenase activity plays an important role in PAR4 and GPVI-mediated platelet reactivity

Following initial platelet activation, arachidonic acid is metabolised by cyclooxygenase-1 and 12-lipoxygenase (12-LOX). While the role of 12-LOX in the platelet is not well defined, recent evidence suggests that it may be important for regulation of platelet activity and is agonist-specific in the manner in which it regulates platelet function. Using small molecule inhibitors selective for 12-LOX and 12-LOX-deficient mice, the role of 12-LOX in regulation of human platelet activation and thrombosis was investigated. Pharmacologically inhibiting 12-LOX resulted in attenuation of platelet aggregation, selective inhibition of dense versus alpha granule secretion, and inhibition of platelet adhesion under flow for PAR4 and collagen. Additionally, 12-LOX-deficient mice showed attenuated integrin activity to PAR4-AP and convulxin compared to wild-type mice. Finally, platelet activation by PARs was shown to be differentially dependent on COX-1 and 12-LOX with PAR1 relying on COX-1 oxidation of arachidonic acid while PAR4 being more dependent on 12-LOX for normal platelet function. These studies demonstrate an important role for 12-LOX in regulating platelet activation and thrombosis. Furthermore, the data presented here provide a basis for potentially targeting 12-LOX as a means to attenuate unwanted platelet activation and clot formation.

Original article: low regulatory volume decrease rate in platelets from ischemic patients: a possible role for hepoxilin a(3) in thrombogenicity

Hepoxilin-A(3) (Hx-A(3)) is produced by platelets in response to shear-stress. It has an antithrombotic effect on platelets. A low Hx-A(3) level may contribute to the high thrombogenic state that exists in patients with acute coronary syndromes. Since we have previously demonstrated that the regulatory volume decrease (RVD) of human platelets exposed to hypotonic solutions is controlled by Hx-A(3) it is possible that the RVD rate reflects Hx-A(3) activity. In this study, the RVD rate of platelets taken from a healthy control group (n=21) was compared to that of patients with chronic ischemic heart disease (n=23), acute ischemic heart disease (n = 24) and acute myocardial infarction (MI, n = 29). The RVD rate of the control group was significantly higher than the other three groups (P < 0.001). The addition of 100 nM of Hx-A, to the platelets of eight patients with MI increased their RVD rate to that of the controls. Patients with diabetes mellitus or hypertension have the lowest RVD rates. Medications such as aspirin, heparin, and streptokinase did not affect the Hx-A(3) activity of platelets obtained from patients with ischemic heart disease. The results of the present study indicate that patients with acute ischemia may have a low level of platelet Hx-A(3) activity. This possible low level of Hx-A, activity may be associated with a failure to develop an antithrombotic reaction to the shear-stress forces generated during acute ischemia.

The hepoxilins and some analogues: a review of their biology

The hepoxilin pathway was discovered over two decades ago. Products in this pathway are derived through the 12S-lipoxygenase/hepoxilin synthase enzyme system and contain intrinsic biological activity. This activity relates to the reorganization of calcium and potassium ions within the cell, and in inflammation and insulin secretion. Although the natural hepoxilins are chemically unstable, chemical analogues (PBTs) have been synthesized with chemical and biological stability. The PBTs antagonize the natural hepoxilins. The PBTs showed bioavailability, excellent tolerance and stability in vivo. In proof of principle studies in vivo in animal models, the PBTs have shown actions as anti-inflammatory agents, anti-thrombotic agents, anti-cancer agents and anti-diabetic agents. These studies demonstrate the effectiveness of the base structure of the hepoxilin (and PBT) molecule and serve as an excellent framework for the design and preparation of second-generation compounds with improved pharmaceutical properties as therapeutics for the above-mentioned diseases.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Cell volume regulation: physiology and pathophysiology

Cell volume perturbation initiates a wide array of intracellular signalling cascades, leading to protective and adaptive events and, in most cases, activation of volume-regulatory osmolyte transport, water loss, and hence restoration of cell volume and cellular function. Cell volume is challenged not only under physiological conditions, e.g. following accumulation of nutrients, during epithelial absorption/secretion processes, following hormonal/autocrine stimulation, and during induction of apoptosis, but also under pathophysiological conditions, e.g. hypoxia, ischaemia and hyponatremia/hypernatremia. On the other hand, it has recently become clear that an increase or reduction in cell volume can also serve as a specific signal in the regulation of physiological processes such as transepithelial transport, cell migration, proliferation and death. Although the mechanisms by which cell volume perturbations are sensed are still far from clear, significant progress has been made with respect to the nature of the sensors, transducers and effectors that convert a change in cell volume into a physiological response. In the present review, we summarize recent major developments in the field, and emphasize the relationship between cell volume regulation and organism physiology/pathophysiology.

Biological role of hepoxilins: upregulation of phospholipid hydroperoxide glutathione peroxidase as a cellular response to oxidative stress?

The 12S-lipoxygenase (12S-LOX) pathway of arachidonic acid (AA) metabolism is bifurcated at 12(S)-hydroperoxy-5Z,8Z,10E (12S-HpETE) in the reduction route to form 12S-hydroxy-eicosatetraenoic acid (12S-HETE) and in 8(S/R)-hydroxy-11(S),12S-trans-epoxyeicosa-5Z,9E,14Z-trienoic acid (HXA3) synthase pathway, previously known as isomerization route, to form hepoxilins. Earlier we showed that the HXA3 formation is restricted to cellular systems devoid of hydroperoxide reducing enzymes, e.g. GPxs, thus causing a persistent oxidative stress situation. Here, we show that HXA3 at as low as 100 nM concentration upregulates phospholipid hydroperoxide glutathione peroxidase (PHGPx) mRNA and protein expressions, whereas other metabolites of AA metabolism 12S-HpETE and 12S-HETE failed to stimulate the PHGPx. Moreover, the decrease in 12S-HpETE below a threshold value of the hydroperoxide tone causes both suppression of the overall 12S-LOX activity and a shift from HXA3 formation towards 12S-HETE formation. We therefore propose that under persistent oxidative stress the formation of HXA3 and the HXA3-induced upregulation of PHGPx constitute a compensatory defense response to protect the vitality and functionality of the cell.

Novel eicosanoid pathways: the discovery of prostacyclin/6-keto prostaglandin F1alpha and the hepoxilins

This article reviews a lecture I was honored to present at the Leon Wolfe Symposium in Montreal on March 25, 2004. The lecture described my research career, which started with my interaction with Wolfe at the Montreal Neurological Institute as a postdoctoral fellow and research associate and was followed by additional research discoveries after I left Montreal for my first academic position at the Research Institute, The Hospital for Sick Children and University of Toronto. The article consists of two parts. The first part involves the discovery (in Wolfe's laboratory) of a new pathway of arachidonic acid, in which a bicyclic prostanoid structure (later called prostacyclin by John Vane and his group) was described, and its further development in Toronto, which led to the discovery of the conversion of the bicyclic prostanoid into 6-keto prostaglandin F1alpha. The second part deals with the hepoxilin pathway, a pathway I discovered during a sabbatical leave in Japan with Professor Shozo Yamamoto, which was followed by a stay of several months in the laboratory of Professor Bengt Samuelsson in Sweden. I deal with the historical aspects of both pathways and end with interesting novel aspects of hepoxilin stable antagonist analogs in the treatment of solid tumors in experimental animals.

Hepoxilin A3 synthase

Hepoxilins constitute a group of 12S-hydroperoxyeicosatetraenoic acid (12S-HpETE)-derived epoxy-hydroxy fatty acids that have been detected in various cell types and tissues. Although hepoxilin A3 (HXA3) exhibits a myriad of biological activities, its biosynthetic mechanism was not investigated in detail. Here we review the isolation, cloning, and characterization of a leukocyte-type 12S-lipoxygenase (12S-LOX) from rat insulinoma cells RINm5F, which exhibits an intrinsic hepoxilin A3 synthase activity. Confirmation for this observation was achieved by coimmunoprecipitation of HXA3 synthase activity with an anti-leukocyte 12S-LOX antibody, preparation of recombinant rat 12S-LOX enzyme from RINm5F cells, and assay of HXA3 synthase activity therein. Site-directed mutagenesis studies performed on rat 12S-LOX showed that 12-lipoxygenating enzyme species exhibit a strong HXA3 synthase activity that is impaired when the positional specificity of arachidonic acid is altered in favor of 15-lipoxygenation. Inasmuch as cellular glutathione peroxidases (cGPx and PHGPx) and HXA3 synthase compete for the same substrate 12S-HpETE, it can be proposed that the overall activity of glutathione peroxidases, representing the overall peroxide tone, finely tunes the rate of HXA3 formation.

Epoxide hydrolases: their roles and interactions with lipid metabolism

The epoxide hydrolases (EHs) are enzymes present in all living organisms, which transform epoxide containing lipids by the addition of water. In plants and animals, many of these lipid substrates have potent biologically activities, such as host defenses, control of development, regulation of inflammation and blood pressure. Thus the EHs have important and diverse biological roles with profound effects on the physiological state of the host organisms. Currently, seven distinct epoxide hydrolase sub-types are recognized in higher organisms. These include the plant soluble EHs, the mammalian soluble epoxide hydrolase, the hepoxilin hydrolase, leukotriene A4 hydrolase, the microsomal epoxide hydrolase, and the insect juvenile hormone epoxide hydrolase. While our understanding of these enzymes has progressed at different rates, here we discuss the current state of knowledge for each of these enzymes, along with a distillation of our current understanding of their endogenous roles. By reviewing the entire enzyme class together, both commonalities and discrepancies in our understanding are highlighted and important directions for future research pertaining to these enzymes are indicated.

Polymorphonuclear Cell Transmigration Induced byPseudomonas aeruginosaRequires the Eicosanoid Hepoxilin A3

Lung inflammation resulting from bacterial infection of the respiratory mucosal surface in diseases such as cystic fibrosis and pneumonia contributes significantly to the pathology. A major consequence of the inflammatory response is the recruitment and accumulation of polymorphonuclear cells (PMNs) at the infection site. It is currently unclear what bacterial factors trigger this response and exactly how PMNs are directed across the epithelial barrier to the airway lumen. An in vitro model consisting of human PMNs and alveolar epithelial cells (A549) grown on inverted Transwell filters was used to determine whether bacteria are capable of inducing PMN migration across these epithelial barriers. A variety of lung pathogenic bacteria, including Klebsiella pneumoniae, Escherichia coli, and Pseudomonas aeruginosa are indeed capable of inducing PMN migration across A549 monolayers. This phenomenon is not mediated by LPS, but requires live bacteria infecting the apical surface. Bacterial interaction with the apical surface of A549 monolayers results in activation of epithelial responses, including the phosphorylation of ERK1/2 and secretion of the PMN chemokine IL-8. However, secretion of IL-8 in response to bacterial infection is neither necessary nor sufficient to mediate PMN transepithelial migration. Instead, PMN transepithelial migration is mediated by the eicosanoid hepoxilin A3, which is a PMN chemoattractant secreted by A549 cells in response to bacterial infection in a protein kinase C-dependent manner. These data suggest that bacterial-induced hepoxilin A3 secretion may represent a previously unrecognized inflammatory mechanism occurring within the lung epithelium during bacterial infections.

Regulation of the cellular content of the organic osmolyte taurine in mammalian cells

Change in the intracellular concentration of osmolytes or the extracellular tonicity results in a rapid transmembrane water flow in mammalian cells until intracellular and extracellular tonicities are equilibrated. Most cells respond to the osmotic cell swelling by activation of volume-sensitive flux pathways for ions and organic osmolytes to restore their original cell volume. Taurine is an important organic osmolyte in mammalian cells, and taurine release via a volume-sensitive taurine efflux pathway is increased and the active taurine uptake via the taurine specific taurine transporter TauT decreased following osmotic cell swelling. The cellular signaling cascades, the second messengers profile, the activation of specific transporters, and the subsequent time course for the readjustment of the cellular content of osmolytes and volume vary from cell type to cell type. Using Ehrlich ascites tumor cells, NIH3T3 mouse fibroblasts and HeLa cells as biological systems, it is revealed that phospholipase A2-mediated mobilization of arachidonic acid from phospholipids and subsequent oxidation of the fatty acid via lipoxygenase systems to potent eicosanoids are essential elements in the signaling cascade that is activated by cell swelling and leads to release of osmolytes. The cellular signaling cascade and the activity of the volume-sensitive taurine efflux pathway are modulated by elements of the cytoskeleton, protein tyrosine kinases/phosphatases, GTP-binding proteins, Ca2+/calmodulin, and reactive oxygen species and nucleotides. Serine/threonine phosphorylation of the active taurine uptake system TauT or a putative regulator, as well as change in the membrane potential, are important elements in the regulation of TauT activity. A model describing the cellular sequence, which is activated by cell swelling and leads to activation of the volume-sensitive efflux pathway, is presented at the end of the review.

The rat leukocyte-type 12-lipoxygenase exhibits an intrinsic hepoxilin A3 synthase activity

Hepoxilins are biologically relevant eicosanoids formed via the 12-lipoxygenase pathway of the arachidonic acid cascade. Although these eicosanoids exhibit a myriad of biological activities, their biosynthetic mechanism has not been investigated in detail. We examined the arachidonic acid metabolism of RINm5F rat insulinoma cells and found that they constitutively express a leukocyte-type 12S-lipoxygenase. Moreover, we observed that RINm5F cells exhibit an active hepoxilin A(3) synthase that converts exogenous 12S-HpETE (12S-5Z,8-Z,10E,14Z-12-hydro(pero)xy-eicosa-5,8,10,14-tetraenoic acid) or arachidonic acid predominantly to hepoxilin A(3). 12S-lipoxygenase and hepoxilin A(3) synthase activities were co-localized in the cytosol; immunoprecipitation with an anti-12S-lipoxygenase antibody co-precipitated the two catalytic activities. These data suggested that hepoxilin A(3) synthase activity may be considered an intrinsic catalytic property of the leukocyte-type 12S-lipoxygenase. To test this hypothesis we cloned the leukocyte-type 12S-LOX from RINm5F cells, expressed it in Pichia pastoris, and found that the recombinant enzyme exhibited both 12S-lipoxygenase and hepoxilin A(3) synthase activities. The recombinant human platelet-type 12S-lipoxygenase and the porcine leukocyte-type 12S-lipoxygenase also exhibited hepoxilin A(3) synthase activity. In contrast, the native rabbit reticulocyte-type 15S-lipoxygenase did not convert 12S-HpETE to hepoxilin isomers. These data suggest that the positional specificity of lipoxygenases may be crucial for this catalytic function. This hypothesis was confirmed by site-directed mutagenesis studies that altered the positional specificity of the rat leukocyte-type 12S- and the rabbit reticulocyte-type 15-lipoxygenase. In summary, it may be concluded that naturally occurring 12S-lipoxygenases exhibit an intrinsic hepoxilin A(3) synthase activity that is minimal in lipoxygenase isoforms with different positional specificity.

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

Mechanisms of cell volume regulation and possible nature of the cell volume sensor

In animal organisms, cell volume undergoes dynamic changes in many physiological and pathological processes. To protect themselves against lysis and apoptosis and to maintain an optimal concentration of intracellular enzymes and metabolites, most animal cells actively regulate their volume. In the present review, we shortly summarize the data on ion transport mechanisms involved in regulatory volume decrease (RVD) and regulatory volume increase (RVI) with an emphasis on unresolved aspects of this problem such as: (i) how cells sense their volume changes; (ii) what signals are generated upon cell volume alterations; and (iii) how these signals are transferred to the ion transport systems executing cell volume regulation.

Evidence for the presence of phospholipid hydroperoxide glutathione peroxidase in human platelets: implications for its involvement in the regulatory network of the 12-lipoxygenase pathway of arachidonic acid metabolism

The 12-lipoxygenase pathway of arachidonic acid metabolism in platelets and other cells is bifurcated into a reduction route yielding 12-hydroxyeicosatetraenoic acid (12-HETE) and an isomerization route forming hepoxilins. Here we show for the first time the presence of phospholipid hydroperoxide glutathione peroxidase (PHGPx) protein and its activity in platelets. The ratio of the activity of PHGPx to that of cytosolic glutathione peroxidase (GPx-1) was consistently found to be approx. 1:60 in platelets and UT7 megakaryoblasts. Moreover, short-lived PHGPx mRNA was detected in megakaryocytes but not in platelets. Carboxymethylation of selenium-containing glutathione peroxidases by iodoacetate, which results in the inactivation of PHGPx and GPx-1 without inhibition of 12-lipoxygenase, markedly altered the pattern of arachidonic acid metabolism in human platelets. Whereas the formation of 12-HETE was inhibited by 80%, a concomitant accumulation of 12-hydroperoxyeicosatetraenoic acid (12-HpETE) by two orders of magnitude as well as the formation of hepoxilins A(3) and B(3) were observed. The formation of hepoxilins also occurred when 12-HpETE was added to untreated platelets. In selenium-deficient UT7 cells, which were devoid of GPx-1 but not of PHGPx, the reduction of 12-HPETE was retained, albeit with a lower rate than in control cells containing GPx-1. We therefore believe that both GPx-1 and PHGPx are involved in the regulatory network of the 12-lipoxygenase pathway in platelets and other mammalian cells. Moreover, the diminution of hydroperoxide tone in platelets incubated with arachidonic acid leads primarily to the formation of 12-HETE, whereas the increase in hydroperoxide tone (a situation found under oxidative stress or selenium deficiency or on incubation with 12-HPETE) partly diverts the 12-lipoxygenase pathway from the reduction route to the isomerization route, thus resulting in the formation of hepoxilins.

Epoxide hydrolases: biochemistry and molecular biology

Epoxides are organic three-membered oxygen compounds that arise from oxidative metabolism of endogenous, as well as xenobiotic compounds via chemical and enzymatic oxidation processes, including the cytochrome P450 monooxygenase system. The resultant epoxides are typically unstable in aqueous environments and chemically reactive. In the case of xenobiotics and certain endogenous substances, epoxide intermediates have been implicated as ultimate mutagenic and carcinogenic initiators Adams et al. (Chem. Biol. Interact. 95 (1995) 57-77) Guengrich (Properties and Metabolic roles 4 (1982) 5-30) Sayer et al. (J. Biol. Chem. 260 (1985) 1630-1640). Therefore, it is of vital importance for the biological organism to regulate levels of these reactive species. The epoxide hydrolases (E.C. 3.3.2. 3) belong to a sub-category of a broad group of hydrolytic enzymes that include esterases, proteases, dehalogenases, and lipases Beetham et al. (DNA Cell Biol. 14 (1995) 61-71). In particular, the epoxide hydrolases are a class of proteins that catalyze the hydration of chemically reactive epoxides to their corresponding dihydrodiol products. Simple epoxides are hydrated to their corresponding vicinal dihydrodiols, and arene oxides to trans-dihydrodiols. In general, this hydration leads to more stable and less reactive intermediates, however exceptions do exist. In mammalian species, there are at least five epoxide hydrolase forms, microsomal cholesterol 5,6-oxide hydrolase, hepoxilin A(3) hydrolase, leukotriene A(4) hydrolase, soluble, and microsomal epoxide hydrolase. Each of these enzymes is distinct chemically and immunologically. Table 1 illustrates some general properties for each of these classes of hydrolases. Fig. 1 provides an overview of selected model substrates for each class of epoxide hydrolase.

Hyposmotically induced amino acid release from the rat cerebral cortex: role of phospholipases and protein kinases

In an evaluation of the contribution of swelling-induced amino acid release, through the regulatory volume decrease (RVD) process, to cerebral ischemic injury, studies of the role of phospholipases and protein kinases in the response to hyposmotic stress were undertaken using an in vivo rat cortical cup model. Hyposmotic stress induced significant releases of aspartate, glutamate, glycine, phosphoethanolamine, taurine and GABA from the rat cerebral cortex. Taurine release was most affected, exhibiting a greater than 9-fold increase during the hyposmotic stimulus. The phospholipase A2 (PLA2) inhibitors 4-bromophenacyl bromide (1 microM) and 7,7-dimethyleicosadienoic acid (5 microM) had no significant effects on hyposmotically induced amino acid release. AACOCF3 (50 microM), an inhibitor of cytosolic PLA2 decreased taurine release to 84% of DMSO controls. The release of the other amino acids was not affected. The phospholipase C inhibitor U73122 (5 microM) had no significant effects on amino acid release. The protein kinase C (PKC) inhibitor chelerythrine (5 microM) significantly reduced hyposmotically induced taurine release to 72% of saline controls but had no significant effects on the other amino acids. Stimulation of PKC with phorbol 12-myristate, 13-acetate (10 microM) did not significantly change taurine, glutamate, glycine or phosphethanolamine release. The releases of aspartate and GABA were enhanced 2 to 3 fold. Phorbol 12,13-didecanoate (10 microM), another potent stimulator of PKC, significantly increased taurine release to 122% of DMSO controls. The releases of aspartate, glutamate and glycine were enhanced 2.5 to 3.5 fold. Similarly, stimulation of protein kinase A with forskolin (100 microM) significantly increased taurine, aspartate, and glycine release 1.5- to 2-fold compared to DMSO controls. In summary, phospholipases may play a minor role in volume regulation. These studies also support the hypothesis that protein kinases play a modulatory role in the RVD response. The results show that although RVD may play a role, additional mechanisms, including phospholipase activation, must be involved in the ischemia-evoked release of excitotoxic amino acids.

Formation of 14,15-Hepoxilins of the A3 and B3 Series through a 15-Lipoxygenase and Hydroperoxide Isomerase Present in Garlic Roots

We report herein for the first time the formation by freshly grown garlic roots and the structural characterization of 14,15-epoxide positional analogs of the hepoxilins formed via the 15-lipoxygenase-induced oxygenation of arachidonic acid. These compounds are formed through the combined actions of a 15(S)-lipoxygenase and a hydroperoxyeicosatetraenoic acid (HPETE) isomerase. The compounds were formed when either arachidonic acid or 15-HPETE were used as substrates. Both the "A"-type and the "B"-type products are formed although the B-type compounds are formed in greater relative quantities. Chiral phase high performance liquid chromatography analysis confirmed the formation of hepoxilins from 15(S)- but not 15(R)-HPETE, indicating high stereoselectivity of the isomerase. Additionally, the lipoxygenase was of the 15(S)-type as only 15(S)-hydroxyeicosatetraenoic acid was formed when arachidonic acid was used as substrate. The structures of the products were confirmed by gas chromatography-mass spectrometry of the methyl ester trimethylsilyl ether derivatives as well as after characteristic epoxide ring opening catalytically with hydrogen leading to dihydroxy products. That 15(S)-lipoxygenase activity is of functional importance in garlic was shown by the inhibition of root growth by BW 755C, a dual cyclooxygenase/lipoxygenase inhibitor and nordihydroguaiaretic acid, a lipoxygenase inhibitor. Additional biological studies were carried out with the purified intact 14(S), 15(S)-hepoxilins, which were investigated for hepoxilin-like actions in causing the release of intracellular calcium in human neutrophils. The 14,15-hepoxilins dose-dependently caused a rise in cytosolic calcium, but their actions were 5-10-fold less active than 11(S), 12(S)-hepoxilins derived from 12(S)-HPETE. These studies provide evidence that 15(S)-lipoxygenase is functionally important to normal root growth and that HPETE isomerization into the hepoxilin-like structure may be ubiquitous; the hepoxilin-evoked release of calcium in human neutrophils, which is receptor-mediated, is sensitive to the location within the molecule of the hydroxyepoxide functionality.

Cardiac cell volume: crystal clear or murky waters? A comparison with other cell types

The osmolarity of bodily fluids is strictly controlled so that most cells do not experience changes in osmotic pressure under normal conditions, but osmotic changes can occur in pathological states such as ischemia, septic shock, and diabetic coma. The primary effect of a change in osmolarity is to acutely alter cell volume. If the osmolarity around a cell is decreased, the cell swells, and if increased, it shrinks. In order to tolerate changes in osmolarity, cells have evolved volume regulatory mechanisms activated by osmotic challenge to normalise cell volume and maintain normal function. In the heart, osmotic stress is encountered during a period of myocardial ischemia when metabolites such as lactate accumulate intracellularly and to a certain degree extracellularly, and cause cell swelling. This swelling may be exacerbated further on reperfusion when the hyperosmotic extracellular milieu is replaced by normosmotic blood. In this review, we describe the theory and mechanisms of volume regulation, and draw on findings in extracardiac tissues, such as kidney, whose responses to osmotic change are well characterised. We then describe cell volume regulation in the heart, with particular emphasis on the effect of myocardial ischemia. Finally, we describe the consequences of osmotic cell swelling for the cell and for the heart, and discuss the implications for antiarrhythmic drug efficacy. Using computer modelling, we have summated the changes induced by cell swelling, and predict that swelling will shorten the action potential. This finding indicates that cell swelling is an important component of the response to ischemia, a component modulating the excitability of the heart.

[New automatic test for the study of platelet response to variations in osmotic pressure: application in the evaluation of the viability of platelet concentrates]

Studying the osmotic resistance or swelling of platelets has often been suggested as a global test to assess the viability of those cells. A number of authors have also analysed the behaviour of platelets in hypotonic media by a variety of complementary methods (cell count, morphology, determinations of substances released, photometric measurement of aggregation induced by aggregating agents, etc). Most studies are currently based on the so-called "osmotic shock response" test, which measures according to time the light transmitted through platelet-rich plasma (PRP) after dilution in distilled water. In this study, the authors describe a new automated and reproducible test using slow dialysis to assess platelet osmotic resistance. The "Fragilimeter", a device initially described by the authors to characterise RBC fragility, has been adapted to the study of platelet osmotic behaviour. The variations in light transmission through a platelet suspension according to NaCl concentration are linked to the change in cellular volume and lysis and characterise the viability of the cells. The results obtained with normal platelets revealed the good reproducibility of the technique. The osmotic resistance is evaluated for two parameters: anticoagulant (citrate, EDTA) and cellular concentration. The test was applied to quality control of stored platelet concentrates for transfusion, prepared with different cell separators.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Influence of sex on sodium-proton exchange-dependent swelling of platelets from patients with essential hypertension

OBJECTIVE

To determine whether sex influences the rate of sodium-proton (Na+-H+)-exchange-dependent swelling of platelets from patients with essential hypertension and normotensive controls.

METHOD

Platelet swelling was detected by measuring the change in optical density of platelet suspensions added to sodium propionate buffer, pH 6.7, at 37 degrees C. We studied 56 subjects, 28 men and 28 women, each group containing 14 normotensive and 14 hypertensive subjects. The groups were well matched for sex, ethnicity and age.

RESULTS

That platelet swelling was dependent on Na+-H+ exchange was demonstrated by performing blockade by 5-(N,N-hexamethylene)-amiloride and by measuring its dependence on extracellular Na+ concentration. The rate of swelling of platelets from hypertensive men [(14.2 +/- 0.9) x 10(-3)/s] was higher than that of those from normotensive men [(10.1 +/- 0.5) x 10(-3)/s], normotensive women [(10.0 +/- 0.5) x 10(-3)/s] and hypertensive [(11.1 +/- 0.8) x 10(-3)/s] women. The interaction between sex and hypertension was significant (P < 0.05 by analysis of variance).

CONCLUSIONS

Sex influences the effect of hypertension on the rate of swelling of platelets exposed to sodium propionate (pH 6.7).

Cell volume regulated transporters of compatible osmolytes

Virtually all cells respond to hypertonicity by accumulating certain small organic solutes (compatible osmolytes) that, in contrast to intracellular ions, do not perturb macromolecular function. Several important compatible osmolytes are accumulated by coupled transport. Transcription of genes encoding these cotransporters is increased by hypertonicity and a tonicity-responsive enhancer element has been identified. When cells return to an iso-osmotic environment, osmolytes are rapidly lost through a pathway that current evidence indicates may be a volume-sensitive chloride channel.

Rapid activation of the heat shock transcription factor, HSF1, by hypo-osmotic stress in mammalian cells

Osmoregulation is important to living organisms for survival in responding to environmental changes of water and ionic strength. We demonstrated here for the first time that exposure of HeLa cells to a hypotonic medium (30% growth medium and 70% water) prominently induced the binding activity of the heat shock transcription factor (HSF). Pretreatment of cells with cycloheximide did not inhibit the induction of HSF-binding activity, indicating that the mechanisms of induction are independent of new protein synthesis. The magnitude of hypo-osmotic stress-induced HSF-binding activity was comparable with that induced by heat shock. The induction, as monitored by gel-mobility-shift assay, occurred within 5 min of hypo-osmotic stress and persisted at least up to 4 h in HeLa cells under the hypotonic conditions. Addition of sorbitol to the hypotonic medium abolished HSF activation. Hypo-osmotic stress-induced HSF binding could also be demonstrated in HeLa cells maintained in simple sorbitol solution by decreasing the sorbitol concentration from 300 mM to 200 mM or less. Competition analysis suggests that the effects of hypo-osmotic stress on HSF-binding activity was specific. Cross-linking experiments and Western-blot analysis demonstrated that hypo-osmotic stress induced trimerization of human heat shock factor 1 (HSF1) in intact HeLa cells, suggesting that trimer formation of HSF1 was responsible for inducing HSF-binding activity in hypo-osmotically stressed cells. However, unlike heat shock response, the activation of HSF by hypo-osmotic stress did not lead to accumulation of hsp70 mRNA in HeLa cells.

Hepoxilins: a review on their enzymatic formation, metabolism and chemical synthesis

This article reviews published evidence describing the enzymatic and nonenzymatic formation and the routes of metabolism of the hepoxilins. Also treated are the major approaches used for the chemical synthesis of these compounds and for some of their analogs.

Membrane mechanisms and intracellular signalling in cell volume regulation

Recent work on selected aspects of the cellular and molecular physiology of cell volume regulation is reviewed. First, the physiological significance of the regulation of cell volume is discussed. Membrane transporters involved in cell volume regulation are reviewed, including volume-sensitive K+ and Cl- channels, K+, Cl- and Na+, K+, 2Cl- cotransporters, and the Na+, H+, Cl-, HCO3-, and K+, H+ exchangers. The role of amino acids, particularly taurine, as cellular osmolytes is discussed. Possible mechanisms by which cells sense their volumes, along with the sensors of these signals, are discussed. The signals are mechanical changes in the membrane and changes in macromolecular crowding. Sensors of these signals include stretch-activated channels, the cytoskeleton, and specific membrane or cytoplasmic enzymes. Mechanisms for transduction of the signal from sensors to transporters are reviewed. These include the Ca(2+)-calmodulin system, phospholipases, polyphosphoinositide metabolism, eicosanoid metabolism, and protein kinases and phosphatases. A detailed model is presented for the swelling-initiated signal transduction pathway in Ehrlich ascites tumor cells. Finally, the coordinated control of volume-regulatory transport processes and changes in the expression of organic osmolyte transporters with long-term adaptation to osmotic stress are reviewed briefly.

Initiation of RVD response in human platelets: mechanical-biochemical transduction involves pertussis-toxin-sensitive G protein and phospholipase A2

Platelets revert hypotonic-induced swelling by the process of regulatory volume decrease (RVD). We have recently shown that this process is under the control of endogenous hepoxilin A3. In this work, we investigated the mechanical-biochemical transduction that leads to hepoxilin A3 formation. We demonstrate that this process is mediated by pertussis-toxin-sensitive G protein, which activates Ca(2+)-insensitive phospholipase A2, and the sequential release of arachidonic acid. This conclusion is supported by the following observations: (i) RVD response is blocked selectively by the phospholipase A2 inhibitors manoalide and bromophenacyl-bromide (0.2 and 5 microM, respectively) but not by phospholipase C inhibitors. The addition of arachidonic acid overcame this inhibition; (ii) extracellular Ca2+ depletion by EGTA (up to 10 mM) does not affect RVD; (iii) intracellular Ca2+ depletion by BAPTA-AM (100 microM) inhibits RVD but not hepoxilin A3 formation, as tested by the RVD reconstitution assay; (iv) RVD is inhibited by the G-protein inhibitors, GDP beta S (1 microM) and pertussis toxin (1 ng/ml). This inhibition is overcome by addition of arachidonic acid or hypotonic cell-free eluate that contains hepoxilin A3; (v) NaF, 1 mM, induces hepoxilin A3 formation, tested by the RVD reconstitution assay; and (vii) GDP beta S inhibits hepoxilin A3 formation associated with flow. Therefore, it seems that G proteins are involved in the initial step of the mechanical-biochemical transduction leading to hepoxilin A3 formation in human platelets.