Search

Rationale: The signal intensity of a given molecule across a tissue section when measured using mass spectrometry imaging (MSI) is prone to changes caused by the molecular heterogeneity across the surface of the tissue. Here we propose a strategy to investigate these effects using electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) on a single high-resolution mass spectrometry (HRMS) platform., Methods: A rat was administered with a single inhaled dose of a compound and sacrificed 1 h after dosing. Sections were prepared from the excised frozen lung and analysed using MALDI, liquid extraction surface analysis (LESA) nano-ESI-MS and nano-ESI liquid chromatography (LC)/MS. The ESI and MALDI ion sources were mounted either side of the ion transfer system of the same Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer., Results: MALDI MSI clearly demonstrated widespread distribution of the dosed molecule throughout the lung, with the exception of a non-lung section of tissue on the same sample surface. Comparison of the lipid signals across the sample indicated a change in signal between the lung and the adipose tissue present on the same section. Use of ESI and MALDI, with and without an internal standard, supported the evaluation of changes in the signal of the dosed molecule across the tissue section., Conclusions: The results demonstrate the successful application of a dual ion source HRMS system to the systematic evaluation of data from MALDI MSI, used to determine the distribution of an inhaled drug in the lung. The system discussed is of great utility in investigating the effects of ion suppression and evaluating the quantitative and qualitative nature of the MSI data., (Copyright © 2014 John Wiley & Sons, Ltd.)

Aim: We investigated how GW800644, the first pharmacologically selective murine peroxisome proliferator-activated receptor δ (PPARδ) agonist, affects energy balance, glucose homeostasis and fuel utilization by muscle in obese mice., Methods: Potencies were determined in transactivation assays. Oral glucose tolerance was determined after 14 and 22 days' administration (10 mg/kg body weight, twice daily) to Lep(ob)/Lep(ob) mice. Food intake and energy expenditure were measured during a 26-day experiment, and plasma metabolites and 2-deoxyglucose uptake in vivo at termination. Palmitate oxidation and 2-deoxyglucose uptake by isolated soleus muscles were measured after 14 (in lean and obese mice) and 26 days., Results: GW800644 activated murine PPARδ (EC(50) 2 nM), but caused little to no activation of PPARα or PPARγ up to 10 µM. It did not increase liver weight. GW800644 reduced food intake and body weight in obese mice after 8 days. It did not affect resting energy expenditure, but, compared to pair-fed mice, it increased the response to a β(3)-adrenoceptor agonist. It improved glucose tolerance. GW800644, but not pair-feeding, reduced plasma glucose, insulin and triglyceride concentrations. It increased 2-deoxyglucose uptake in vivo in adipose tissue, soleus muscle, heart, brain and liver, and doubled 2-deoxyglucose uptake and palmitate oxidation in isolated soleus muscle from obese but not lean mice., Conclusions: PPARδ agonism reduced food intake and independently elicited metabolic effects that included increased responsiveness to β(3)-adrenoceptor stimulation, increased glucose utilization and fat oxidation in soleus muscle of Lep(ob)/Lep(ob) but not lean mice and increased glucose utilization in vivo in Lep(ob)/Lep(ob) mice., (© 2011 Blackwell Publishing Ltd.)

Background: The peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors and members of the nuclear receptor superfamily. The PPAR family consists of three members: PPARalpha, PPARgamma, and PPARdelta. PPARdelta controls the transcription of genes involved in multiple physiological pathways, including cellular differentiation, lipid metabolism and energy homeostasis. The receptor is expressed almost ubiquitously, with high expression in liver and skeletal muscle. Although the physiological ligands of PPARdelta remain undefined, a number of high affinity synthetic ligands have been developed for the receptor as a therapeutic target for type 2 diabetes mellitus, dyslipidemia and the metabolic syndrome., Methods: In this study, the metabolic role of PPARdelta activation has been investigated in liver, skeletal muscle, blood serum and white adipose tissue from ob/ob mice using a high affinity synthetic ligand and contrasted with PPARgamma activation. To maximize the analytical coverage of the metabolome, (1)H-nuclear magnetic resonance ((1)H-NMR) spectroscopy, gas chromatography-mass spectrometry (GC-MS) and ultra performance liquid chromatography-mass spectrometry (UPLC-MS) were used to examine metabolites from tissue extracts., Results: Analysis by multivariate statistics demonstrated that PPARdelta activation profoundly affected glycolysis, gluconeogenesis, the TCA cycle and linoleic acid and alpha-linolenic acid essential fatty acid pathways., Conclusions: Although activation of both PPARdelta and PPARgamma lead to increased insulin sensitivity and glucose tolerance, PPARdelta activation was functionally distinct from PPARgamma activation, and was characterized by increased hepatic and peripheral fatty acid oxidative metabolism, demonstrating the distinctive catabolic role of this receptor compared with PPARgamma.

Objective: Exercise increases fatty acid oxidation (FAO), improves serum high density lipoprotein cholesterol (HDLc) and triglycerides (TG), and upregulates skeletal muscle peroxisome proliferator activated receptor (PPAR)delta expression. In parallel, PPARdelta agonist-upregulated FAO would induce fatty-acid uptake (via peripheral lipolysis), and influence HDLc and TG-rich lipoprotein particle metabolism, as suggested in preclinical models., Methods and Results: Healthy volunteers were allocated placebo (n=6) or PPARdelta agonist (GW501516) at 2.5 mg (n=9) or 10 mg (n=9), orally, once-daily for 2 weeks while hospitalized and sedentary. Standard lipid/lipoproteins were measured and in vivo fat feeding studies were conducted. Human skeletal muscle cells were treated with GW501516 in vitro and evaluated for lipid-related gene expression and FAO. Serum TG trended downwards (P=0.08, 10 mg), whereas TG clearance post fat-feeding improved with drug (P=0.02). HDLc was enhanced in both treatment groups (2.5 mg P=0.004, 10 mg P<0.001) when compared with the decrease in the placebo group (-11.5+/-1.6%, P=0.002). These findings complimented in vitro cell culture results whereby GW501516 induced FAO and upregulated CPT1 and CD36 expression, in addition to a 2-fold increase in ABCA1 (P=0.002). However, LpL expression remained unchanged., Conclusions: This is the first report of a PPARdelta agonist administered to man. In this small study, GW501516 significantly influenced HDLc and TGs in healthy volunteers. Enhanced in vivo serum fat clearance, and the first demonstrated in vitro upregulation in human skeletal muscle fat utilization and ABCA1 expression, suggests peripheral fat utilization and lipidation as potential mechanisms toward these HDL:TG effects.

The pyruvate dehydrogenase complex occupies a central and strategic position in muscle intermediary metabolism and is primarily regulated by phosphorylation/dephosphorylation. The identification of multiple isoforms of pyruvate dehydrogenase kinase (PDK1-4) and pyruvate dehydrogenase phosphatase (PDP1-2) has raised intriguing new possibilities for chronic pyruvate dehydrogenase complex control. Experiments to date suggest that PDK4 is the major isoenzyme responsible for changes in pyruvate dehydrogenase complex activity in response to various different metabolic conditions. Using a cultured human skeletal muscle cell model system, we found that expression of both PDK2 and PDK4 mRNA is upregulated in response to glucose deprivation and fatty acid supplementation, the effects of which are reversed by insulin treatment. In addition, insulin directly downregulates PDK2 and PDK4 mRNA transcript abundance via a phosphatidylinositol 3-kinase-dependent pathway, which may involve glycogen synthase kinase-3 but does not utilize the mammalian target of rapamycin or mitogen-activated protein kinase signalling pathways. In order to further elucidate the regulation of PDK, the role of the peroxisome proliferators-activated receptors (PPAR) was investigated using highly potent subtype selective agonists. PPARalpha and PPARdelta agonists were found to specifically upregulate PDK4 mRNA expression, whereas PPARgamma activation selectively decreased PDK2 mRNA transcript abundance. PDP1 mRNA expression was unaffected by all conditions analysed. These results suggest that in human muscle, hormonal and nutritional conditions may control PDK2 and PDK4 mRNA expression via a common signalling mechanism. In addition, PPARs appear to independently regulate specific PDK isoform transcipt levels, which are likely to impart important metabolic mediation of fuel utilization by the muscle.

Nicotinic acid has been used clinically for over 40 years in the treatment of dyslipidemia producing a desirable normalization of a range of cardiovascular risk factors, including a marked elevation of high density lipoprotein and a reduction in mortality. The precise mechanism of action of nicotinic acid is unknown, although it is believed that activation of a G(i)-G protein-coupled receptor may contribute. Utilizing available information on the tissue distribution of nicotinic acid receptors, we identified candidate orphan receptors. The selected orphan receptors were screened for responses to nicotinic acid, in an assay for activation of G(i)-G proteins. Here we describe the identification of the G protein-coupled receptor HM74 as a low affinity receptor for nicotinic acid. We then describe the subsequent identification of HM74A in follow-up bioinformatics searches and demonstrate that it acts as a high affinity receptor for nicotinic acid and other compounds with related pharmacology. The discovery of HM74A as a molecular target for nicotinic acid may facilitate the discovery of superior drug molecules to treat dyslipidemia.

Apolipoprotein E (apoE) is a 34 kDa glycoprotein with multiple actions that help protect against the development of atherosclerosis. Here, we have assessed the atheroprotective potential of an [E1(-), E3(-), polymerase(-)] adenovirus vector expressing human apoE, comparing intramuscular and intravenous (liver-directed) injections in hypercholesterolaemic apoE-deficient mice (apoE(-/-)). Intramuscular injections resulted in low expression of apoE and afforded no protection against atherogenesis. In contrast, 3 and 7 days after intravenous injections into young (6-8-week-old) apoE(-/-) mice, plasma levels of apoE were elevated and were accompanied by reductions in plasma cholesterol and normalization of lipoprotein profiles. Thereafter, plasma apoE was still detectable up to day 70, but gradually declined, although no humoral immune response was evoked, and there was a return to dyslipoproteinaemia. High levels of the vector genome were still present in livers of treated animals at 70 days, implying that decrease in apoE expression was due to cellular shutdown of the cytomegalovirus promoter. Importantly, liver-directed apoE gene transfer to these young mice retarded progression of atherosclerosis by 38% (treated, 8.21 +/- 1.05%; untreated, 13.26 +/- 0.98%, P < 0.05), during the 70 day study period. Moreover, when 10-month-old apoE(-/-) mice with advanced atherosclerosis were treated with the adenovirus vector, there was clear regression of aortic lesion area by 1 month [24.3 +/- 1.7% compared to 40.7 +/- 2.6% in baseline controls (P < 0.002)]. We conclude that the stability of the adenovirus vector genome in the livers of intravenously treated animals provides an ideal platform to evaluate liver-specific promoters for sustained transgene expression and control of atherosclerotic lesion pathology.

Successful management of cardiovascular (CV) disease and associated metabolic syndromes, such as diabetes, is a major challenge to the clinician. Reducing CV risk factors, such as abnormal lipid profiles, insulin resistance or hypertension is the foundation of such therapy. A relatively new class of therapeutic agent, activators of peroxisome proliferator-activated receptors (PPAR), is poised to make a major impact with regard to several areas of risk factor management. However, there is growing evidence that PPAR agonists may also influence the CV system directly by modulating vessel wall function. These observations suggest that additional benefit, in the treatment of CV disease, may derive not only from the ability of agents to modify risk factors but also to influence directly the cellular mechanisms of disease within the vessel wall. A precedent for this dual action comes from examination of the effects of inhibitors of HMG CoA reductase (statins), where risk factor modulation is accompanied by direct actions on the vessel wall. In this review, we summarize the evidence suggesting that PPAR agonists may directly modulate vessel wall function, and that these may parallel those effects reported recently for the statins.

Atherosclerotic plaques develop in the arterial wall from complex multicellular processes following the early recruitment of circulating monocytes. Infiltration of monocytes is mediated by cell adhesion molecules (CAMs), including vascular cell adhesion molecule-1 (VCAM-1) which is rapidly induced in endothelial cells in response to cytokines. Apolipoprotein E (apo E), a 34-kDa polypeptide, helps protect against atherosclerosis, in part, because apo E phospholipid particles secreted by macrophages may have local protective effects within lesions. Here we have investigated whether purified plasma apo E, complexed with dimyristoyl phosphatidylcholine (DMPC) vesicles, can inhibit cytokine-induced vascular cell adhesion molecule-1 (VCAM-1) expression in human umbilical vein endothelial cells (HUVECs). Expression of VCAM-1 in endothelial cells after exposure to tumour necrosis factor-alpha (TNF-alpha) or interleukin 1beta (IL-1beta) was quantified by ELISA and shown to be partially inhibited by 17beta-estradiol (40-60% inhibition) or by S-nitroso-L-glutathione, a nitric oxide donor (20-25%). However, preincubations with physiological concentrations (10-100 microg protein/ml) of apo E DMPC did not downregulate VCAM-1 expression, even with extended preincubation times. These findings were confirmed using a fluorescence-activated cell sorter (FACS) for analysis which indicated additionally that apo E-DMPC had no effect on sub-populations within the HUVEC cultures. Finally, apo E-DMPC vesicles were also unable to suppress TNF-alpha-induced upregulation of E-selectin or intercellular adhesion molecule-1 (ICAM-1). We conclude that plasma apo E is unlikely to be important in limiting endothelial activation.

One mechanism by which plasma high-density lipoprotein (HDL) may protect against atherogenesis is by inhibiting the oxidation of low-density lipoprotein (LDL). Recent evidence suggests that paraoxonase, an HDL-associated, calcium-dependent enzyme, may be responsible for the antioxidant action of HDL (Mackness et al., Atherosclerosis 1993;104:129; Mackness et al., FEBS Lett 1991;286:152; Watson et al., J Clin Invest 1995;96:2882; Navab et al., Arterio Thromb Vasc Biol 1996;16:831); in particular, paraoxonase activity inhibits the formation of 'minimally oxidized' LDL by hydrolyzing biologically active oxidized phospholipids (Watson et al., J Clin Invest 1995;96:2882; Navab et al., Arterio Thromb Vasc Biol 1996;16:831). However, antioxidant effects of HDL have also been demonstrated under calcium-free conditions, arguing that this enzyme may not be the only mechanism by which HDL inhibits LDL oxidation (Tribble et al., J Lipid Res 1995;36:2580). Here we have evaluated the role of paraoxonase in prevention of LDL oxidation by using HDL subfractions, isolated from human serum or EDTA-plasma, which display markedly different levels of paraoxonase activity; the abilities of modified forms of HDL to prevent LDL oxidation by cultured human (THP-1) macrophages were also assessed. Paraoxonase activity was substantially lower in HDL prepared from plasma compared to serum HDL; moreover, virtually all of the lipoprotein-associated paraoxonase activity was located in the HDL3 fraction, with HDL2 retaining only 1-5% of the total activity. Despite possessing 5-fold differences in paraoxonase activity, HDL3 isolated from plasma or serum was equally effective in inhibiting LDL oxidation by THP-1 macrophages; furthermore, although plasma HDL3 was more protective than plasma HDL2, the latter did significantly inhibit LDL oxidation. Non-paraoxonase antioxidant constituents of plasma HDL3 were investigated further. ApoHDL3, the totally delipidated form of HDL3, was much less effective than native HDL3; when examined individually, purified apolipoprotein A-II gave greater protection than apo A-I, although this effect was not evident in apo A-II-enriched HDL3. Partial delipidation of HDL3, which removes both neutral lipids and alpha-tocopherol, did not significantly diminish its ability to inhibit LDL oxidation by THP-1 macrophages; phospholipid vesicles prepared from partially delipidated HDL3 also inhibited LDL oxidation effectively. We conclude that, in this model of cellular LDL oxidation, the phospholipid fraction of HDL exerts inhibitory effects which are independent of HDL paraoxonase activity.

The formation of macrophage-derived foam cells is central to the development of fatty streaks within the arterial wall, and to the progression of atherosclerosis. The unregulated deposition of cholesteryl esters, as lipid droplets within the cytoplasm of these cells, is responsible for the formation of foam cells; this process is thought to be regulated by the balance between cholesterol esterification, by acyl CoA:cholesterol acyltransferase (ACAT), and hydrolysis, by neutral cholesteryl ester hydrolase (nCEH). This study examines the importance of the balance between these two enzymes in determining the efflux of cholesterol from human (THP-1) macrophages. The presence of modified lipoprotein, or of 25-hydroxycholesterol, markedly increased cholesterol esterification in these cells and these effects were potently inhibited by the presence of the ACAT inhibitor, 447C88. In the absence of HDL, an acceptor particle, there was little or no hydrolysis of the cholesteryl ester pool and no efflux of cholesterol to the extracellular milieu; addition of HDL led to a partial (36%) reduction in cholesteryl esters, an effect which was not enhanced by the inhibition of ACAT. This suggested that the stored cholesteryl esters in human (THP-1) macrophages, unlike those in mouse peritoneal macrophages, were relatively resistant to removal by efflux to HDL. Efflux of newly synthesised free cholesterol from these macrophages was increased by HDL in a saturable manner, suggesting that the lack of reduction of stored cholesteryl esters was due to impaired mobilisation of cholesteryl esters to free cholesterol via nCEH. Indeed, nCEH activity in these macrophages was much lower than in mouse peritoneal macrophages, and appeared to be down-regulated in the presence of 25-hydroxycholesterol or modified lipoproteins; this loss of nCEH activity was prevented by the ACAT inhibitor 447C88. The efflux of stored cholesteryl esters from THP-1 macrophages therefore appears to be limited by the activity of nCEH.

The free cholesterol content of cells can be monitored by the intensity of fluorescence emissions from the polyene antibiotic filipin. In a previous study (Hassall: Cytometry 13:381-388, 1992) using THP-1 macrophages, a decrease in filipin fluorescence in response to increasing concentrations of modified lipoprotein was observed, suggesting a reduction in the free cholesterol content of the cells. In this study, THP-1 macrophages were treated with a number of agents known to modulate cholesterol biosynthesis and cholesterol esterification. Changes in filipin fluorescence emissions were measured by flow cytometry, and correlated with changes in cholesterol biosynthesis measured by incorporation of [14C]acetate into cholesterol. A correlation between decreases in filipin fluorescence and reductions in cholesterol biosynthesis was apparent, even when cholesterol esterification was inhibited. These results suggest that the decreases in filipin fluorescence observed may be due, at least in part, to reduction in cholesterol biosynthesis.

SC-44368 (5-[6-(1-cyclohexyl-1H-tetrazol-5-y)hexyl]-1,8-naphthyridin-2(1H)-one) is a potent and selective competitive inhibitor of platelet cyclic AMP-dependent phosphodiesterase (cAMP-PDE) (Ki: 1.65 μM). For the phosphodiesterase isoenzyms from human platelets SC-44368 shows a 26-fold selectivity (IC50 ratio) for the inhibition of the cAMP-PDE over the cyclic GMP-dependent phosphodiesterase (cGMP-PDE). By comparison, 3-isobutyl-1-methyl-xanthine (IBMX) inhibited the cAMP-PDE and cGMP-PDE from human platelets with approximately equal efficacy. Broad inhibitory activity was evident against human platelet aggregatory responses in vitro. IC50 values of 18.1 ± 5.3 μM (25 nM platelet activating factor, PAF), 17.3 ± 3.0 μM (1.0 μg/ml collagen) and 24.2 ± 10.3 μM (1μM ADP) were obtained against maximum increases in platelet-rich plasma (PRP) light transmission achieved by each agonist. SC-44368 potentiated the prostacyclin-induced increase of intra-platelet cAMP levels but did not potentiate the sodium nitroprusside-induced increase of intraplatelet cGMP levels. In an ex vivo model of platelet aggregation SC-44368 (3 mg/kg, i.v.) produced a potent inhibition of collagen-induced platelet aggregation. SC-44368 produced only weak hypotensive activity in the rat. Thus, SC-44368 is a novel cAMP-PDE inhibitor which possesses potent, broad spectrum anti-aggregatory properties.

A human cell line THP-1 was differentiated into macrophages expressing the scavenger receptor for uptake of modified lipoproteins. The cells were exposed to native low-density lipoprotein (n-LDL), acetylated-low-density lipoprotein (Ac-LDL), oxidised-LDL, or 25-OH cholesterol, leading to the accumulation of cholesteryl esters within the cells. Harvested macrophages were studied using three separate probes: 1) 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (diI)-labelled LDL to study lipoprotein uptake; 2) the lipophilic fluorescent dye Nile Red to study cholesteryl ester accumulation within the cells; and 3) the polyene antibiotic Filipin III to study free cholesterol homeostasis. Cells were analysed using fluorescence flow cytometry and the three signals analysed separately. THP-1 macrophages incubated with diI-labelled modified lipoproteins produced a concentration dependent increase in the fluorescence emissions, consistent with accumulation of the labelled particles. Macrophages exposed to unlabelled modified LDLs were demonstrated, by staining with Nile Red, to accumulate cholesteryl esters within their cytoplasm and to alter their cholesterol content as judged by staining with Filipin. The foam-cell forming macrophage and its response to modified lipoproteins is considered a key step in the development of atherosclerosis. The use of these three probes during the formation of foam-cells in vitro offers a way of studying their behaviour at the single cell level.

The adherence of blood leukocytes to vascular endothelium precedes their diapedesis into the extravascular space. These processes require the expression of adherence glycoproteins on the cell surface of the leukocyte. The relative importance of these adherence molecules is so far poorly understood. However, there is evidence to suggest that a disparity exists between the surface receptor expression of these glycoproteins and leukocyte adherence to vascular endothelial cells in culture. We have investigated the importance of each of the adhesion glycoproteins CD11a, CD11b, and CD11c in mediating the adherence of human monocytes to endothelial cells in culture. We have also investigated the chronological relationship between changes in monocyte adherence to endothelial cells and the surface expression of CD11a, CD11b, and CD11c following stimulation with N-formyl-methionyl-leucyl-phenylalanine (fMLP). The increase in adherence occurred within 1 minute, but declined if monocytes were preincubated with fMLP for up to 30 minutes. The surface expression of adherence molecules demonstrated a significant increase in CD11a and CD11b in the presence of fMLP after 10 min and was maintained while monocyte adherence to endothelium declined. These changes in surface receptor expression were quantitated using an immunolabeling technique. It is suggested that fMLP stimulation of monocyte adherence is unlikely to be solely dependent on increased surface receptor expression of adhesion molecules.

Monocyte-endothelial interactions are of fundamental importance in determining the movement of monocytes from the blood stream into the vessel wall. This study reports that two endothelium-derived factors, nitric oxide and prostacyclin, alter in vitro monocyte behavior. Nitric oxide (greater than 10(-5) M) inhibited monocyte adhesion to porcine aortic endothelial cell monolayers, whereas prostacyclin (10(-9) to 10(-5) M) had no effect. Both nitric oxide and prostacyclin inhibited monocyte chemotaxis stimulated by N-formyl-methionyl-leucyl-phenylalanine and induced dose-dependent increases in intracellular cyclic guanosine monophosphate and cyclic adenosine monophosphate concentrations, respectively. The cell surface expression of the CD11b/CD18 adhesion receptor, a glycoprotein complex known to mediate monocyte intracellular adhesion, was not altered by either nitric oxide or by prostacyclin. Thus, endothelium-derived nitric oxide and prostacyclin may have a physiological role in modulating monocyte-vascular wall interactions. Alterations in this system may contribute to the increased monocyte emigration from the blood stream into the vessel wall observed in atherogenesis.

Leukocyte adherence is mediated by a superfamily of glycoproteins denoted LFA-1 (the lymphocyte function-associated antigen-1), Mac-1 (macrophage antigen-1) and p150,95. The relative importance of these in mediating human monocyte adherence to endothelium, and the biochemical mechanisms which modulate these events, are not understood. In this report, the role of protein kinase C (pkC) in regulating human monocyte adherence to endothelial cells has been investigated. Addition of phorbol 12,13-dibutyrate (PDBu), which specifically stimulates pkC, caused a dose-dependent increase in their adherence to monolayers of bovine aortic endothelial cells. 4 alpha-phorbol didecanoate (4 alpha-PDD), a structural analogue of PDBu which does not stimulate pkC, failed to increase monocyte adhesion. PDBu also produced a dose-dependent increase in the expression of both Mac-1 and p150,95. The pkC-stimulated adherence of monocytes to endothelium was inhibited by the presence of a monoclonal antibody to Mac-1, while monoclonal antibodies to p150,95 and LFA-1 did not influence adherence. It is concluded that monocyte adherence to endothelial cells is regulated through a pkC-dependent mechanism; moreover, this process is mediated primarily via the Mac-1 adhesion glycoprotein.

Sensitisation of human blood platelets by plasma low-density lipoproteins (LDL) is thought to involve an initial interaction between LDL and the platelet surface membrane. When washed, fully responsive platelets were incubated with radio-iodinated LDL, two phases of binding were identified; one due to saturable sites, binding about 1965 ± 177 (mean ± SEM) LDL molecules per platelet, and the other of low affinity but high capacity. The saturable sites could be distinguished from the LDL receptors of nucleated cells by their presence in patients with homozygous familial hypercholesterolaemia, by their resistance to pronase digestion, by their reduced affinity for apolipoprotein E (apoE), and by the absence of a requirement for calcium ions. Binding of LDL by platelets was rapid and apparently mediated by recognition of positively charged arginine and lysine residues within the protein constituent (apoB) of LDL, findings previously reported as characteristics of LDL stimulation of platelet aggregation. Blotting of electrophoretically separated platelet membrane proteins with radio-iodinated LDL revealed a single component, presumably the saturable, high affinity binding site, whose apparent molecular weight of approx. 140 000 was lower than that of the classical LDL receptor. We conclude that occupation of (glyco)proteins on the platelet surface membrane by LDL molecules enhances platelet reactivity.

A new, rapid and reproducible method is described for the quantification of in vitro monocyte adherence in microtitre 96-well ELISA plates. The assay is based on the measurement of the myeloperoxidase activity in monocytes that have adhered to either plastic or bovine/porcine aortic endothelial cells. The assay shows a linear response to monocyte concentrations over the range 5 x 10(4)-5 x 10(5) cells/well. The method provides good discrimination between monocyte and and lymphocytes and thus allows a study of mononuclear cell populations to be made. The assay works equally well with polymorphonuclear cells. The coefficients of variation are: intra-assay, 7%; interassay, 13%.

Aggregation tests were performed on platelet-rich plasma from healthy male volunteers to determine the minimum concentration of adenosine diphosphate (ADP), epinephrine, collagen, or thrombin required to induce secondary aggregation. Platelets were also analyzed for cholesterol and phospholipid, and in some cases their membrane fluidity was determined by fluorescence depolarization of the probe, diphenylhexatriene. Concentrations of the major lipoprotein fractions in the plasma were measured and related to the sensitivity of platelets to the four agonists. Low density lipoprotein (LDL) and total cholesterol concentrations, but not high density lipoproteins (HDL) or very low density lipoproteins (VLDL), were positively correlated with sensitivity to aggregation by epinephrine, but not by other agonists. By arrangement of the lipoprotein concentration into quintiles, the effect of LDL was most striking in the lower two quintiles, where the sensitivity to adrenaline and ADP were much diminished. The middle and upper two quintiles showed a similar sensitivity. Lower platelet cholesterol/phospholipid ratios were also associated with a reduced sensitivity to epinephrine or ADP, but only at the lower end of the range. Membrane microviscosity was correlated negatively with collagen sensitivity and with VLDL cholesterol concentrations, but positively with HDL cholesterol concentrations. Platelet behavior appears, therefore, to be influenced by lipoprotein concentrations within the range found in a healthy population.

Isolated platelets are strongly sensitized by the presence of low-density lipoproteins (LDL) so that they aggregate with very low concentrations of other agonists or exhibit spontaneous aggregation. Prostacyclin (PGI2) is a potent inhibitor of aggregation, but its action was reversed by LDL. This effect of LDL was accompanied by a decrease in the synthesis of cyclic AMP induced by PGI2, but its efficacy depended on the relative concentrations of LDL and PGI2. PGI2 also enhanced the synthesis of cyclic GMP, but this was completely reversed by the presence of LDL. LDL did not remove inhibitory prostaglandins, e.g. E1, from their receptor sites. The lipoproteins also decreased cyclic AMP synthesis induced by forskolin, which has its effect on the GTP-sensitive protein or the catalytic unit of the adenylate cyclase enzyme complex. It is proposed that LDL may act on the enzyme catalytic unit via an inhibitory GTP-sensitive protein or by a separate mechanism which indirectly impedes the production of cyclic AMP.

Addition of prostacyclin (PGI2) temporarily inhibits platelet aggregation and permits the isolation of platelets free from plasma proteins, which have the same sensitivity as those in plasma [Moncada, Radomski & Vargas (1982) Br. J. Pharmacol. 75, 165P]. By using a modification of this technique we have established that platelets isolated from normal subjects aggregate more readily in response to ADP and adrenaline when physiological concentrations of low-density lipoproteins (LDL) are present. At high LDL concentrations spontaneous aggregation occurs. High-density lipoproteins (HDL) and very-low-density lipoproteins (VLDL) had no effect on agonist-induced platelet aggregation at normal concentrations, but HDL sensitized at higher concentrations. These effects by lipoproteins are not accompanied by changes in platelet lipid content. Cyclohexanedione treatment of LDL to modify apolipoproteins appeared to abolish the sensitization effect, indicating that binding to receptors was essential for the effects of LDL. LDL, but not HDL, overcame the inhibitory effect of PGI2 on platelet aggregation, except at very high concentrations of PGI2. PGI2 raised the cyclic AMP content of isolated platelets, but LDL only partially prevented this rise. These results suggest that LDL may have a greater role in platelet aggregation than previously recognized and may also regulate effects of PGI2. These findings may be of relevance to an understanding of cardiovascular diseases.

A new rabbit model of atherosclerosis is described in which several of the features seen in early human atherosclerosis are generated within a period of 7 days. The positioning of a hollow silastic collar around the carotid artery of a cholesterol-fed rabbit results in macrophage and smooth muscle cell infiltration into the arterial subendothelium, foam cell formation and the deposition of extracellular lipid. A time-dependent accumulation of extracellular cholesteryl ester occurs within the arterial wall. Each of these changes occurs in the presence of a morphologically intact endothelium as assessed using light microscopy, scanning and transmission electron microscopy. A high cholesterol diet did not affect the extent of proliferation but exacerbated cholesteryl ester accumulation. It is proposed that the changes induced by the collar may be mediated by obstruction of the adventitial vasa vasorum with the creation of a localised ischaemic region.

It has been reported that lanosterol can sensitize isolated rat platelets to agonists such as ADP and thrombin (4). The purpose of this paper was to determine whether lanosterol had similar effects on human platelets and whether this was achieved by changes in membrane fluidity. Lanosterol did increase the sensitivity of human platelets, particularly to adrenaline and ADP at concentrations as low as 5 mg.L-1 when added from solutions in ethanol. At similar concentrations cholesterol, 4-cholesten-3-one or ethynyloestradiol had either no effect or were inhibitory. Measurement of membrane fluidity with diphenylhexatriene indicated that lanosterol did not affect membrane fluidity. Incubation of platelets with [4C]-mevalonic acid gave rise to a very small incorporation into lanosterol, squalene and farnesol. Sudden activation of the platelets did not accelerate lanosterol synthesis during or after platelet aggregation. It was concluded that lanosterol could only influence platelet behaviour if it came from the plasma. However the concentration of the steroid in both platelets and plasma is ten fold less than that required to sensitise the platelets.

Low-density lipoproteins (LDL) have been shown to cause aggregation of human blood platelets at concentrations above 2 g of protein/l. The secretion of the contents of platelet dense granules was detected, but not that of the lysosomes. LDL gave rise to a mobilization of [3H]arachidonic acid from phospholipids and the appearance of products of the cyclo-oxygenase pathway after only 10 s. LDL-promoted aggregation was inhibited by both aspirin and indomethacin. There was an increase in 3H-labelled diacylglycerols and the phosphorylation of 47 kDa proteins. LDL therefore shares at least some of the mechanisms of stimulus/response coupling with those of other agonists.