Itinerary of high density lipoproteins in endothelial cells

High density lipoprotein (HDL) and its main protein component apolipoprotein A-I (ApoA-I) have multiple anti-atherogenic functions. Some of them are exerted within the vessel wall, so that HDL needs to pass the endothelial barrier. To elucidate their itinerary through endothelial cells (ECs), we labelled ApoA-I and HDL either fluorescently or with 1.4 nm nanogold and investigated their cellular localization by using immunofluorescent microscopy (IFM) and electron microscopy (EM). HDL as well as ApoA-I is taken up by ECs into the same route of intracellular trafficking. Time kinetics and pulse chase experiments revealed that HDL is trafficked through different vesicles. HDL partially co-localized with LDL, albumin, and transferrin. HDL did not co-localize with clathrin and caveolin-1. Fluorescent HDL was recovered at small proportions in early endosomes and endosome to trans-golgi network vesicles but not at all in recycling endosomes, in late endosomes or lysosomes. EM identified HDL mainly in large filled vesicles which however upon IFM did not colocalize with markers of multivesicular bodies or autophagosomes. The uptake or cellular distribution of HDL was altered upon pharmacological interference with cytochalasine D, colchicine and dynasore. Blockage of fluid phase uptake with Amiloride or EIPA did not reduce the uptake of HDL. Neither did we observe any co-localization of HDL with dextran as the marker of fluid phase uptake. In conclusion, HDL and ApoA-I are internalized and trafficked by endothelial cells through a non-classical endocytic route.     

A re-evaluation of the mitogenic effect of serotonin on vascular endothelial cells

Serotonin is an extracellular mediator recognized by seven different types of receptors, thus giving rise to pleiotropic intracellular responses. One of these responses is the activation of proliferation for a number of cell types. The induction of proliferation of otherwise quiescent endothelial cells is a key step of angiogenesis. Previously published work concerning the effect of serotonin on endothelial cell proliferation is controversial. The present work is aimed to re-evaluate the mitogenic role of serotonin on endothelial cells, since a pro-angiogenic role for serotonin could be hypothesized if its mitogenic potential on these cells were confirmed. By using three different types of endothelial cells and three experimental approaches, we demonstrate that serotonin cannot be considered a general mitogen for endothelial cells.     

Thrombin mitogenic responses and protein phosphorylation are different in cultured human endothelial cells derived from large and microvessels

It is well established that thrombin induces various biological responses in endothelial cells derived from large vessels. However, little is known about the effects of thrombin on the microvasculature. Protein phosphorylation may be one of the mechanisms by which an extracellular stimulus initiates cellular events like proliferation. Therefore, we have compared the effects of either human alpha-thrombin or phorbol esters (TPA) on the proliferation or protein phosphorylation in endothelial cells derived from large vessels (umbilical vein, HUVEC) or microvessels (omental tissue, HOMEC). In HOMEC, thrombin did not stimulate cell proliferation and protein phosphorylation while TPA slightly reduced the cell proliferation and induced the phosphorylation of a 27-kDa protein. In contrast, in HUVEC, thrombin or TPA markedly enhanced the cell proliferation and stimulated the phosphorylation of a 59-kDa protein. These data indicate that (i) endothelial cells from large and small vessels respond differently to thrombin and (ii) there is a complex and as yet unclear relationship between the proliferation and the protein phosphorylation induced by thrombin.     

The effect of fibrin on cultured vascular endothelial cells

The normal cobblestone monolayer architecture of cultured vascular endothelium becomes rapidly disorganized after contact of the cell layer with a fibrin clot. The cells of a confluent endothelial monolayer separate into individual migratory cells in 4–6 hr after contact with fibrin. The effect is reversible in that removal of the fibrin clot results in resumption of the normal morphology within about 2 hr. No other cell type tested exhibits the same change in organization when exposed to fibrin. A similar morphological change in endothelium does occur after the cell layer is overlaid with a collagen fibril gel but a gel of methylcellulose has no effect. It is proposed that the change in behavior of endothelial cells in response to contact with fibrin may represent a cellular component of fibrinolysis. The implications of this finding for the pathophysiology of disease states involving intravascular fibrin deposition are discussed.     

High density lipoproteins and the growth of vascular endothelial cells in serum-free medium

Summary Low density bovine vacular endothelial cell cultures maintained on dishes coated with an extracellular matrix can be grown in serum-free Dulbecco's modified Eagle's medium supplemented with high density lipoprotein (HDL) and transferrin. Such cultures do not require insulin. Early passage cultures exposed to HDL and transferrin grew as well as cultures exposed to optimal serum concentrations and could be passaged repeatedly in total absence of serum. A requirement for fibroblast growth factor to ensure an optimal growth could be observed only with late-passage cultures. The present results suggest strongly that HDL is involved in supporting the proliferation of vascular endothelial cells in vitro. This may be important for our understanding of the biological role of HDL “in vivo”. This work was supported by Grants HL 23678 and 20192 from the National Institutes of Health, Bethesda, MD.     

Protein phosphorylation in cultured endothelial cells

We have investigated the protein phosphorylation systems present in cultured bovine aortic and pulmonary artery endothelial cells. The cells contain cyclic AMP-dependent protein kinase, three calcium/calmodulin-dependent protein kinases, protein kinase C, and at least one tyrosine kinase. No cyclic GMP-dependent protein kinase activity was found. The cells also contained numerous substrates for cyclic AMP-dependent protein kinase and protein kinase C. Fewer substrates were found for the calcium/calmodulin-dependent protein kinases. There was little difference between either protein kinase activities or substrates when pulmonary artery endothelium was compared to aortic endothelium grown under similar culture conditions. It is likely that these various protein kinases and their respective substrate proteins are involved in mediating several of the actions of the hormones and drugs which affect the vascular endothelium.     

Methylmercury-injury effect on tube formation by cultured human vascular endothelial cells

The effect of methylmercury chloride (MeHg) on growth and tube formation by cultured human umbilical vein endothelial cells (HUVECs) was investigated. HUVECs were collected by enzymatic digestion with collagenase. Precultivation of HUVECs with MeHg at concentrations of 1.0–50.0 mol/L exerted negligible effects on the viable cell number, while the viable cell number was slightly reduced at 100 mol/L and fell to zero at concentrations exceeding 500.0 mol/L MeHg. The viable cell number was depressed in a concentration-dependent manner. Tube formation was studied by culturing the cells on gelled basement membrane matrix (Matrigel). Treatment of HUVECs with 0.1–5.0 mol/L MeHg for 24 h inhibited tube formation dose-dependently. Fetal bovine serum (FBS) increased tube formation in a dose-dependent manner, with half-maximum stimulation of tube formation at approximately 3.4% FBS. The length of tube formation decreased time-dependently at concentrations of 0.1 and 1.0 mol/L MeHg. Pretreatment of Matrigel with 1 mol/L MeHg before the cell seeding reduced the tube formation by HUVECs. These results suggest that the growth and tube formation by HUVECs is susceptible to MeHg cytotoxicity, and that MeHg could be injurious to endothelial cell function.Abbreviations MeHg methylmercury chloride - HUVECs human umbilical vein endothelial cells     

Paracrine mitogenic effect of human endothelial progenitor cells: role of interleukin-8

Endothelial progenitor cells (EPCs) play an important role in repair of vascular injury and neovascularization. Molecular mechanisms underlying vascular effects of EPCs are not fully understood. The present study was designed to test the hypothesis that human EPCs exert a strong paracrine mitogenic effect on mature endothelial cells. Levels of interleukin-8 (IL-8) were significantly higher in conditioned medium (CM) collected from EPCs than in CM derived from mature endothelial cells [umbilical vein endothelial cells (HUVECs) and coronary artery endothelial cells (CAECs)]. CM of EPCs stimulated proliferation of HUVECs and CAECs. This mitogenic effect was partially inhibited by IL-8-neutralizing antibody. In contrast, CM of HUVECs and CAECs had a weak or no mitogenic effect on mature endothelial cells. Our results demonstrate significantly higher levels of IL-8 secretion by human EPCs than by mature endothelial cells. IL-8 appears to be an important mediator of the paracrine mitogenic effect of EPCs.     

High density lipoproteins reduce the uptake of low density lipoproteins by human endothelial cells in culture.

Endothelial cells, explanted from human umbilical veins and cultured, maintained morphological characteristics of vascular endothelium. When exposed to human serum lipoproteins, the cells bound and took up low density lipoproteins in preference to high density lipoproteins. High density lipoproteins reduced markedly the uptake of low density lipoproteins and affected surface binding to a lesser extent. These data suggest that the different levels of high density lipoprotein encountered in normal plasma of males and females could modulate differently the transendothelial transport of low density lipoproteins and provide a possible explanation for the lesser severity of atheromatosis in the aortic intima of premenopausal females.     

Oxidant-sensitive protein phosphorylation in endothelial cells

Reactive oxygen is an important regulator of vascular cell biology; however, the mechanisms involved in transducing signals from oxidants in endothelial cells are poorly defined. Because protein phosphorylation is a major mechanism for signal ransduction, cultured aortic endothelial cells were exposed to nonlethal concentrations of H₂O₂ to examine oxidant-sensitive changes in phosphorylation state. Addition of H₂O₂ increases the phosphorylation of the heat shock protein 27 (HSP27) within 2 min. This response is maximal by 20 min and remains constant for more than 45 min. Levels of intrcellular free Ca²⁺ in endothelial cells did not change following addition of 100 μM H₂O₂, nor did the ability of the cells to respond to bradykinin. H₂O₂-induced phosphorylations were either not affected or were slightly increased in cells pretreated with PKC inhibitors (H-8, staurosporin, or calphostin c). Two-dimensional analysis of phosphoproteins from homogenates of ³²P-labeled cells revealed that phorbol myristate acetate (PMA) did not cause the same degree of HSP27 phosphorylation as H₂O₂. Simultaneous addition of 10 ηM PMA and 50 μM H₂O₂ decreased the oxidant-stimulated phoshorylation of the most acidic HSP27 isoform. These data suggest that signal transduction for H₂O₂-sensitive endothelial cell responses are not only independent of PKC, but may also be suppressed by the action of the kinase.     

Triacylglycerol-rich lipoproteins trigger the phosphorylation of extracellular-signal regulated kinases in vascular cells

Postprandial triacylglycerol-rich lipoproteins (TRL) have been implicated in the pathophysiology of atherosclerosis, but the intracellular processes by which TRL could affect vascular function are still unknown. Incubation of TRL obtained at 2 h postprandial period with vascular smooth muscle cells (VSMC) produced a tyrosine phosphorylation of the extracellular signal-regulated kinases 1 and 2 (ERK1 and ERK2) that belong to the mitogen-activated protein kinase (MAPK) family. The activation of ERK1 and ERK2 had a maximum at 15 min, returned to baseline by 60 min, and was partially depleted after incubation of cells with a MAPKK inhibitor (PD 098059). In addition, postprandial TRL did competent VSMC for DNA replication through a MAPK pathway. These effects were dependent of the lipid composition of TRL. Our observations suggest that postprandial TRL can trigger activation of the MAPK pathway and induce a mitogenic response in VSMC in a lipid-dependent fashion.     

The effect of serum lipoproteins on cholesterol content and sterol exchange in cultured human endothelial cells.

Cultured human endothelial cells preincubated with the infranatant of human serum increased their content of cholesterol when subsequently exposed to low density lipoproteins (LDL) as compared to control cultures further incubated in the presence of infranatant only. Replacing LDL with high density lipoproteins (HDL) resulted in no change in the cellular cholesterol content compared to the control. The addition of HDL did not influence the increase in cellular cholesterol content mediated by LDL. HDL stimulated the efflux of endogenously synthesized 14C-labelled sterols compared to the infranatant fraction, whereas LDL had only a slight effect. Cells preincubated with whole serum did not change their cholesterol content when subsequently exposed to LDL, compared to cultures further incubated in presence of whole serum. Replacing whole serum (during the final incubation) with infranatant, resulted in a decrease of the cellular cholesterol content, which was not influenced by further addition of HDL.     

Human plasma cholesteryl ester transfer protein enhances the transfer of cholesteryl ester from high density lipoproteins into cultured HepG2 cells

The role of human plasma cholesteryl ester transfer protein (CETP) in the cellular uptake of high density lipoprotein (HDL) cholesteryl ester (CE) was studied in a liver tumor cell line (HepG2). When HepG2 cells were incubated with [3H]cholesteryl ester-labeled HDL3 in the presence of increasing concentrations of CETP there was a progressive increase in cell-associated radioactivity to levels that were 2.8 times control. The CETP-dependent uptake of HDL-CE was found to be saturated by increasing concentrations of both CETP and HDL. The CETP-dependent uptake of CE radioactivity increased continuously during an 18-h incubation. In contrast to the effect on cholesteryl ester, CETP failed to enhance HDL protein cell association or degradation. Enhanced uptake of HDL cholesteryl ester was shown for the d greater than 1.21 g/ml fraction of human plasma, partially purified CETP, and CETP purified to homogeneity, but not for the d greater than 1.21 g/ml fraction of rat plasma which lacks cholesteryl ester transfer activity. HDL cholesteryl ester entering the cell under the influence of CETP was largely degraded to free cholesterol by a process inhibitable by chloroquine. CETP enhanced uptake of HDL [3H]CE in cultured smooth muscle cells and to a lesser extent in fibroblasts but did not significantly influence uptake in endothelial cells or J774 macrophages. These experiments show that, in addition to its known role in enhancing the exchange of CE between lipoproteins, plasma CETP can facilitate the in vitro selective transfer of CE from HDL into certain cells.     

Scavenger receptor CD36 mediates uptake of high density lipoproteins in mice and by cultured cells

The mechanisms of HDL-mediated cholesterol transport from peripheral tissues to the liver are incompletely defined. Here the function of scavenger receptor cluster of differentiation 36 (CD36) for HDL uptake by the liver was investigated. CD36 knockout (KO) mice, which were the model, have a 37% increase (P = 0.008) of plasma HDL cholesterol compared with wild-type (WT) littermates. To explore the mechanism of this increase, HDL metabolism was investigated with HDL radiolabeled in the apolipoprotein (¹²⁵I) and cholesteryl ester (CE, [³H]) moiety. Liver uptake of [³H] and ¹²⁵I from HDL decreased in CD36 KO mice and the difference, i. e. hepatic selective CE uptake ([³H]¹²⁵I), declined (–33%, P = 0.0003) in CD36 KO compared with WT mice. Hepatic HDL holo-particle uptake (¹²⁵I) decreased (–29%, P = 0.0038) in CD36 KO mice. In vitro, uptake of ¹²⁵I-/[³H]HDL by primary liver cells from WT or CD36 KO mice revealed a diminished HDL uptake in CD36-deficient hepatocytes. Adenovirus-mediated expression of CD36 in cells induced an increase in selective CE uptake from HDL and a stimulation of holo-particle internalization. In conclusion, CD36 plays a role in HDL uptake in mice and by cultured cells. A physiologic function of CD36 in HDL metabolism in vivo is suggested.     

Protective role of intracellular glutathione against oxidized low density lipoprotein in cultured endothelial cells

We examined the role of intracellular glutathione (GSH) in the defense of endothelial cells against oxidized low density lipoprotein (OX-LDL). Incubation of cultured bovine endothelial cells with OX-LDL produced a loss of intracellular GSH, followed by lysis. A decrease in the cellular stores of GSH by treating the endothelial cells with buthionine sulfoximine, an irreversible inhibitor of gamma-glutamylcysteine synthetase, increased the susceptibility of endothelial cells to lysis by OX-LDL. In contrast, an increase in cellular GSH level by treatment with L-2-oxothiazolidine-4-caboxylate, an effective intracellular cysteine delivery agent, reduced the toxicity of OX-LDL. These findings suggest that intracellular GSH plays an important role in the defense of endothelial cells against OX-LDL, and that the mechanism of OX-LDL toxicity is related to the depletion of intracellular GSH.     

Dual effect of hypochlorite in the modification of high density lipoproteins

HDL-cholesterol levels are inversely correlated to the risk of cardiovascular disease. In recent years the concept that not only the quantity, but also the quality of HDL is related to their atheroprotective function has gained momentum. In fact several studies have showed that HDL can shift their properties from anti-atherogenic to pro-atherogenic upon chemical or enzymatic "modification". However, not all kind of modifications affect the antiatherogenic properties of HDL. For example, tyrosylation of HDL improves its ability to remove cholesterol from cultured cells and inhibits mice atherosclerotic lesion formation; oxidation of HDL(3) with 15-lipoxygenase or with copper ions for short time induce the formation of pre-β-migrating particles that are highly effective as cholesterol acceptors from lipid laden cells. Myeloperoxidase modifies HDL and apoA-I and reduces their ability to promote ABCA1-mediated cholesterol efflux. In the present study we show that modification with low concentration HOCl (a myeloperoxidase product) induces the formation of pre-β-migrating particles, thus improving the function of HDL in the reverse cholesterol transport, without affecting the anti-inflammatory activity. At higher HOCl concentration, pre-β-migrating particles were not detectable and the anti-inflammatory properties of HDL were lost. These findings suggest that during early phases of inflammation, when a low HOCl concentration is generated, changes in HDL occur that increase their ability to remove cholesterol and sparing anti-inflammatory properties; later during acute inflammation, when higher HOCl concentration are present changes in HDL occur that severely decrease their ability to remove cholesterol from macrophages and to protect endothelial cells from pro-inflammatory stimuli.     

Vasoconstrictor-induced protein-tyrosine phosphorylation in cultured vascular smooth muscle cells

In cultured rat aortic smooth muscle cells, angiotensin II induced tyrosine phosphorylation of at least 9 proteins with molecular masses of 190, 117, 105, 82, 79, 77, 73, 45 and 40 kDa in time- and dose-dependent manners. Other vasoconstrictors such as [Arg]vasopressin, 5-hydroxytryptamine and norepinephrine induced the tyrosine phosphorylation of the same set of proteins as angiotensin II. The tyrosine phosphorylation of these proteins was mimicked by the protein kinase C-activating phorbol ester, phorbol 12 myristate 13-acetate, and the Ca2+ ionophore, ionomycin. These results demonstrate that the vasoconstrictors stimulate the tyrosine phosphorylation of several proteins in vascular smooth muscle cells and suggest that the tyrosine phosphorylation reactions are the events distal to the activation of protein kinase C and Ca2+ mobilization in the intracellular signalling pathways of the vasoconstrictors.