Dexamethasone effects on microvascular endothelial cell lipid composition

Modeling of full-length Piezo1 suggests importance of the proximal N-terminus for dome structure

Piezo1 forms a mechanically activated calcium-permeable nonselective cation channel that is functionally important in many cell types. Structural data exist for C-terminal regions, but we lack information about N-terminal regions and how the entire channel interacts with the lipid bilayer. Here, we use computational approaches to predict the three-dimensional structure of the full-length Piezo1 and simulate it in an asymmetric membrane. A number of novel insights are suggested by the model: 1) Piezo1 creates a trilobed dome in the membrane that extends beyond the radius of the protein, 2) Piezo1 changes the lipid environment in its vicinity via preferential interactions with cholesterol and phosphatidylinositol 4,5-bisphosphate (PIP₂) molecules, and 3) cholesterol changes the depth of the dome and PIP₂ binding preference. In vitro alteration of cholesterol concentration inhibits Piezo1 activity in a manner complementing some of our computational findings. The data suggest the importance of N-terminal regions of Piezo1 for dome structure and membrane cholesterol and PIP₂ interactions.

Role of glucocorticoids in neutrophil and endothelial adhesion molecule expression and function

Glucocorticoids are very effective inhibitors of both the acute and chronic inflammatory response. In this study the hypothesis that glucocorticoids inhibit an early component of the inflammatory response, neutrophil adhesion to endothelium, by down-regulation of adhesion molecules on neutrophils or endothelium was examined. No effect of dexamethasone on neutrophil adhesion to endothelium or of antigen expression by neutrophils or endothelium was found. The mechanism of action of glucocorticoids in the inflammatory response is probably not mediated by alterations in adhesion molecules.

Corticosteroid effect on Caco-2 cell lipids depends on cell differentiation

Previous studies from our laboratory have indicated that secondary hyperaldosteronism affects phospholipids of rat colonic enterocytes. To assess whether this represents a direct effect of mineralocorticoids on enterocytes, the role of aldosterone and dexamethasone in the regulation of lipid metabolism was examined in Caco-2 cells during development of their enterocyte phenotype. Differentiation of Caco-2 cells was associated with increased levels of triglycerides (TG) and cholesteryl esters (CE), a decreased content of cholesterol and phospholipids and changes in individual phospholipid classes. The phospholipids of differentiated cells had a higher content of n-6 polyunsaturated fatty acids (PUFA) and lower amounts of monounsaturated (MUFA) and saturated fatty acids than subconfluent undifferentiated cells. Differentiated cells exhibited a higher ability to incorporate [3H]arachidonic acid (AA) into cellular phospholipids and a lower ability for incorporation into TG and CE. Incubation of subconfluent undifferentiated cells with aldosterone or dexamethasone was without effect on the content of lipids, their fatty acids and [3H]AA incorporation. In contrast, aldosterone treatment of differentiated cells diminished the content of TG, increased the content of phospholipids and modulated their fatty acid composition. The percentage of n-6 and n-3 PUFA in phospholipids was increased and that of MUFA decreased, whereas no changes in TG were observed. The incorporation of [3H]AA into phospholipids was increased and into TG decreased and these changes were blocked by spironolactone. Treatment of differentiated cells with dexamethasone increased their CE content but no effect was identified upon other lipids, their fatty acid composition and on the incorporation of [3H]AA. As expected for the involvement of corticosteroid hormones the mineralocorticoid and glucocorticoid receptors were identified in Caco-2 cells by RT-PCR. The results suggest that aldosterone had a profound influence on lipid metabolism in enterocytes and that its effect depends on the stage of differentiation. The aldosterone-dependent changes occurring in phospholipids and their fatty acid composition may reflect a physiologically important phenomenon with long-term consequences for membrane structure and function.

Lipid composition of cultured endothelial cells in relation to their growth

Human endothelial cells in culture were examined in different growth conditions. The human endothelial cell line, EA.hy 926 cell line, was used and cells were studied either in exponential growth phase, at confluence, or growth-arrested by serum deprivation. Phospholipids were separated and analyzed by high-performance thin-layer chromatography, and their fatty acids were quantified by gas-liquid chromatography. No significant differences in the phospholipid distributions were found between exponentially growing and confluent endothelial cells in which phosphatidylcholine (PC) represented the major phospholipid. In comparison, serum-deprived cells exhibited higher proportions of sphingomyelin and lower content of PC. We also found that among the total lipids, cholesterol level for dividing endothelial cells was lower than for cells growth-arrested either by serum deprivation or by contact inhibition at confluence. The global fatty acid distribution was not affected by the growth conditions. Thus, oleate (18:1 n-9 and 18:1 n-7), palmitate (C16:0), and stearate (C18:0) were the main components of endothelial cell membranes. However, the fatty acid distributions obtained from each phospholipid species differed with the growth status. Altogether, the data indicated that subtle modulations of endothelial cell metabolism appear upon cell growth. The resulting membrane-dependent cellular functions such as cholesterol transport and receptor activities can be expected to be relevant for lipid trafficking within the vessel wall in vitro and in vivo.

Increased lipid fluidity in synaptosomes from brains of hyperosmolal rats

Taurine, a product of sulfur amino acid metabolism, is important in cerebral osmoregulation. To understand the adaptive changes in transport which accompany different hyperosmolal states, we determined lipid composition and fluorescence anisotropy of synaptosomal liposomes from rats with chronic hypernatremic dehydration (CHD), streptozocin-induced (STZ) diabetes, and insulin treated diabetes. Induction of CHD increased serum osmolality, and enhanced in vitro synaptosomal taurine uptake (P < 0.01, n = 3, vs. control). Fluorescence anisotropy studies showed that the fluidity of lipids from CHD synaptosomes was higher than control (P < 0.05, n = 3). STZ-diabetes resulted in hyperglycemia, increased serum osmolality, and stimulated synaptosomal taurine uptake (P < 0.01, n = 3, vs. control). Insulin treatment of diabetic rats restored serum osmolality and taurine transport to control values. The fluidity of diabetic rat brain synaptosomal lipids was significantly higher than control (P < 0.05, n = 3); fluidity was normalized by insulin administration to diabetic rats. Total fatty acid, cholesterol, and cholesterol/phospholipid molar ratio of CHD, STZ, and insulin treated diabetic rats were similar to control. However, the ratio of saturated to unsaturated fatty acids was decreased in hyperosmolal states. This suggests that adaptive increases in cerebral taurine transport during hyperosmolality may result from a direct effect on membrane composition that alters fluidity and permits enhanced transmembrane flux of osmoprotective molecules.

Lipid fluidity modulates platelet aggregation and agglutination in vitro

To determine the effect of altered membrane fluidity on platelet aggregation/agglutination, fresh, washed human platelets were treated with A2C, a cyclopropyl fatty acid ester which is known to enhance mobility of intrinsic membrane bilayer constituents and increase membrane fluidity. Fluorescence polarization studies demonstrated A2C incubation time- and concentration-dependent increases in platelet membrane fluidity (decreased fluorescence anisotropy). Preincubation with A2C was associated with diminished collagen, thrombin and ristocetin-induced platelet aggregation/agglutination. Aggregation/agglutination was diminished by 93 +/- 5% for collagen (0.2 mg/ml), 53 +/- 3% for thrombin (1.0 U/ml) and 85 +/- 9% for ristocetin (1.1 mg/ml). These data suggest that membrane fluidity is involved in the regulation of platelet function.

Fibroblast growth factor upregulates PGG/H synthase in rabbit microvascular endothelial cells by a glucocorticoid independent mechanism

The present study was undertaken to determine the effects of acidic fibroblast growth factor (aFGF) on eicosanoid synthesis in microvessel endothelial cells derived from rabbit left ventricular muscle (RCME cells). We observed that aFGF increased AA conversion to PGE2 in a time- and dose-dependent manner, and the stimulatory effect was abolished by actinomycin D and cycloheximide. Acidic FGF increased the recovery of PGG/H synthase activity following aspirin treatment, suggesting an action on de novo PGG/H synthase synthesis. Acidic FGF increased the incorporation of [35S] methionine into a 70 kD immunoreactive PGG/H synthase band. PGG/H synthase synthesis following aspirin treatment was also increased by transforming growth factor beta, while epidermal growth factor basic FGF and platelet derived growth factor were without effect. In addition, the actions of aFGF on de novo PGG/H synthase were compared in several endothelial preparations. Acidic FGF treatment of aspirin treated endothelial cells from rabbit lung microvessels and small pulmonary artery and from human lung microvessels all showed an increase in PGG/H synthase recovery. In contrast, similar treatment of human umbilical vein endothelial cells was without effect. Pretreatment of RCME cells with dexamethasone (1 microM) did not alter the aFGF induction of PGG/H synthase activity. We conclude that aFGF stimulates PGE2 production by a mechanism that includes the de novo synthesis of PGG/H synthase. This mechanism appears to be distinct from previously described glucocorticoid sensitive translational controls of PG synthase synthesis by epidermal growth factor in smooth muscle and mesangial cells.

Sodium fluoride induces phosphoinositide hydrolysis, Ca2+ mobilization, and prostacyclin synthesis in cultured human endothelium: further evidence for regulation by a pertussis toxin-insensitive guanine nucleotide-binding protein

The role of guanine nucleotide-binding proteins in the induction of prostacyclin synthesis by stimulated endothelial cells is incompletely understood. We report that sodium fluoride (NaF), a potent activator of cellular guanine nucleotide-binding proteins, affected time- and concentration-dependent generation of prostacyclin (PGI2) by cultured human umbilical vein endothelial cells without evidence of cellular toxicity detected by 51Cr or lactate dehydrogenase release. PGI2 synthesis by NaF-stimulated endothelial cells was associated with increases in arachidonate release, phosphoinositide hydrolysis, generation of inositol phosphates, and accumulation of diacylglycerol. These responses to NaF, as well as alpha-thrombin-mediated responses, were not dependent upon the availability of extracellular free Ca2+ but were associated with the mobilization of stored intracellular Ca2+ detected by the luminescence of the photoprotein aequorin. Neither PGI2 synthesis nor Ca2+ responses following alpha-thrombin or NaF stimulation were inhibited by pretreatment of cells with the islet activating protein from Bordetella pertussis but were significantly attenuated by the G protein inhibitor GDP beta S in permeabilized cells. Our results are compatible with a model wherein NaF directly activates a phosphoinositidase-linked guanine nucleotide regulatory protein, Gp, in human umbilical vein endothelial cell monolayers. This activation results in phosphoinositide hydrolysis, Ca2+ mobilization, arachidonate release, and subsequent functional activation, assessed by PGI2 release. Biologically relevant agonists such as alpha-thrombin may exert their influence on arachidonate metabolism, in part, by promoting receptor-dependent activation of this G protein.

Glucocorticoid-mediated alterations in fluidity of rabbit cardiac muscle microvessel endothelial cell membranes: influences on eicosanoid release

Experiments were conducted to determine effects of the synthetic glucocorticoid, dexamethasone, on the lipid fluidity of cultured rabbit cardiac muscle microvessel endothelial cells and the possible role(s) for altered fluidity in the steroid inhibition of cellular eicosanoid production. Following a sixteen hour exposure to 10(-7) M dexamethasone, membranes prepared from treated cells exhibited a decreased fluidity compared to their control counterparts, as assessed by steady-state fluorescence polarization techniques using 1,6-diphenyl-1,3,5-hexatriene (DPH). Examination of the effects of temperature on the anisotropy values of DPH using Arrhenius plots revealed consistent differences in the steroid treated cells over the entire temperature range (40-5 degrees C). These dexamethasone-dependent fluidity changes were associated with increases in the cholesterol/phospholipid ratio of membrane lipids. Restoration of membrane fluidity to control values with the fluidizing agent, 2-(2-methoxyethoxy)ethyl-8-(cis- 2-n-octylcyclopropyl)octanoate (A2C), partially reversed dexamethasone induced inhibition of A23187-stimulated eicosanoid release. These observations suggest that at least part of dexamethasone's inhibitory actions on eicosanoid generation in microvessel endothelial cells are mediated by alterations in membrane composition and fluidity.

Characterization of CoA-independent transacylase activity in U937 cells

Coenzyme A-independent transacylase (CoA-IT) mediates the transfer of polyunsaturated fatty acids from the sn-2 position of a donor phospholipid to the sn-2 position of an acceptor lyso-phospholipid. We have characterized this activity in U937 cells, a human monocytic cell line. The microsomes of these cells contained CoA-IT activity which demonstrated a fatty acid preference for transferring arachidonic acid into exogenously added 1-alkyl-2-lyso-GPC. This enzymatic activity was optimum between pH 6.5 and 9, was heat labile and displayed an apparent Km for 1-alkyl-2-lyso-GPC of 0.4 microM. This activity was not dependent on Ca2+, Mg2+, CoA or ATP, was not inhibited by 2-mercaptoethanol nor by addition of product, 1-alkyl-2-acyl-GPC. The activity of this enzyme was not altered by differentiation of U937 cells towards the macrophage with Me2SO. Treatment of U937 cells with dexamethasone had no effect on transacylase activity. The activity of this enzyme was decreased by the serine esterase inhibitors phenylmethyl-sulfonyl fluoride and N-tosyl-L-phenylalanine chloromethyl ketone and by the histidine modifier diethyl pyrocarbonate, suggesting that CoA-IT may belong to a family of acyltransferase enzymes typified by LCAT. CoA-IT activity was not affected by compounds that affect PLA2 activity, such as quinacrine, aristolochic acid and arachidonic acid, suggesting a mechanism of action for CoA-IT different from classical, low molecular weight PLA2 enzymes. In conclusion, U937 cells contain CoA-IT activity and this study extends our previous knowledge of this enzyme by demonstrating the differences between CoA-IT and PLA2 enzymes and suggesting similarities between CoA-IT and LCAT.

Glucocorticoid treatment reduces prostacyclin synthesis in response to limited stimuli

Dexamethasone 21-acetate (DMS 21-A) time- and dose-dependently suppressed bradykinin-stimulated prostacyclin synthesis in porcine aortic endothelial cells. The suppression was more prominent in the presence of pertussis toxin, which by itself could enhance bradykinin-induced prostacyclin synthesis. The DMS 21-A treatment diminished prostacyclin synthesis also in response to vasopressin. In contrast, it did not affect prostacyclin synthesis in response to arachidonic acid or A23187. Melittin-induced prostacyclin synthesis was reduced only at low doses (1-7 x 10(-7) M). The suppression of bradykinin-induced prostacyclin synthesis by DMS 21-A was completely blocked by cycloheximide. DMS 21-A had no effect on the cellular level of lipocortin I protein, but increased the anti-phospholipase A2 activity in EDTA extracts of the cells. These results suggest that the DMS 21-A treatment induces phospholipase A2 inhibitor protein(s) other than lipocortin I and reduces prostacyclin production in response to limited stimuli.

Regulation of eicosanoid synthesis in microvessel endothelium: glucocorticoids do not affect arachidonyl CoA synthetase activity

The properties of acyl coenzyme A (CoA) synthetase activity were characterized in cultured rabbit coronary microvessel endothelial cells. We report here that microvessel endothelial cells contain two long-chain acyl CoA synthetases. One shows activity with a variety of fatty acids, similar to long-chain non-selective fatty acyl CoA synthetases described previously. The other activity was selective for arachidonic acid and other structurally related substrates. Both activities required ATP, Mg2+ and CoA for optimal activity. The arachidonyl CoA and the non-selective acyl CoA synthetases showed different thermolabilities. Arachidonyl CoA formation was inhibited by greater than 50% after 1 min at 45 degrees C, whereas a 15 min heating treatment was necessary to produce the same relative inhibition of oleoyl CoA synthesis. Glucocorticoid pretreatment (10(-7) M dexamethasone) of the RCME cells did not affect the apparent Km or Vmax, nor the fatty acid selectivity for either acyl CoA synthetase. Therefore, although fatty acyl CoA synthetases may be involved in limiting eicosanoid formation, these activities do not appear to be glucocorticoid-responsive.