W. Gray Jerome

Pancreatic Islet Production of Vascular Endothelial Growth Factor-A Is Essential for Islet Vascularization, Revascularization, and Function

To investigate molecular mechanisms controlling islet vascularization and revascularization after transplantation, we examined pancreatic expression of three families of angiogenic factors and their receptors in differentiating endocrine cells and adult islets. Using intravital lectin labeling, we demonstrated that development of islet microvasculature and establishment of islet blood flow occur concomitantly with islet morphogenesis. Our genetic data indicate that vascular endothelial growth factor (VEGF)-A is a major regulator of islet vascularization and revascularization of transplanted islets. In spite of normal pancreatic insulin content and β-cell mass, mice with β-cell–reduced VEGF-A expression had impaired glucose-stimulated insulin secretion. By vascular or diffusion delivery of β-cell secretagogues to islets, we showed that reduced insulin output is not a result of β-cell dysfunction but rather caused by vascular alterations in islets. Taken together, our data indicate that the microvasculature plays an integral role in islet function. Factors modulating VEGF-A expression may influence islet vascularity and, consequently, the amount of insulin delivered into the systemic circulation.

Apolipoprotein A-I structural organization in high-density lipoproteins isolated from human plasma

High-density lipoproteins (HDLs) mediate cholesterol transport and protection from cardiovascular disease. Although synthetic HDLs have been studied for 30 years, the structures of human plasma–derived HDL and its major protein apolipoprotein apoA-I are unknown. We separated normal human HDL into five density subfractions and then further isolated those containing predominantly apoA-I (LpA-I). Using cross-linking chemistry and mass spectrometry, we found that apoA-I adopts a structural framework in these particles that closely mirrors that in synthetic HDL. We adapted established structures for synthetic HDL to generate the first detailed models of authentic human plasma HDL in which apoA-I adopts a symmetrical cage-like structure. The models suggest that HDL particle size is modulated by means of a twisting motion of the resident apoA-I molecules. This understanding offers insights into how apoA-I structure modulates HDL function and its interactions with other apolipoproteins.

Dysfunctional High-Density Lipoprotein in Patients on Chronic Hemodialysis

OBJECTIVES This study examined the functionality of high-density lipoprotein (HDL) in individuals with end-stage renal disease on dialysis (ESRD-HD). BACKGROUND The high rate of cardiovascular disease (CVD) in chronic kidney disease is not explained by standard risk factors, especially in patients with ESRD-HD who appear resistant to benefits of statin therapy. HDL is antiatherogenic because it extracts tissue cholesterol and reduces inflammation. METHODS Cellular cholesterol efflux and inflammatory response were assessed in macrophages exposed to HDL of patients with ESRD-HD or controls. RESULTS HDL from patients with ESRD-HD was dramatically less effective than normal HDL in accepting cholesterol from macrophages (median 6.9%; interquartile range [IQR]: 1.4% to 10.2%) versus control (median 14.9%; IQR: 9.8% to 17.8%; p < 0.001). The profound efflux impairment was also seen in patients with ESRD-HD and diabetes compared with patients with diabetes without renal disease (median 8.1%; IQR: 3.3% to 12.9%) versus control (median 13.6%; IQR: 11.0% to 15.9%; p = 0.009). In vitro activation of cellular cholesterol transporters increased cholesterol efflux to both normal and uremic HDL. HDL of patients with ESRD-HD had reduced antichemotactic ability and increased macrophage cytokine response (tumor necrosis factor-alpha, interleukin-6, and interleukin-1-beta). HDL of patients with ESRD-HD on statin therapy had reduced inflammatory response while maintaining impaired cholesterol acceptor function. Interestingly, impaired HDL-mediated efflux did not correlate with circulating C-reactive protein levels or cellular inflammatory response. CONCLUSIONS These findings suggest that abnormal HDL capacity to mediate cholesterol efflux is a key driver of excess CVD in patients on chronic hemodialysis and may explain why statins have limited effect to decrease CV events. The findings also suggest cellular cholesterol transporters as potential therapeutic targets to decrease CV risk in this population.

Free Cholesterol Accumulation in Macrophage Membranes Activates Toll-Like Receptors and p38 Mitogen-Activated Protein Kinase and Induces Cathepsin K

The molecular events linking lipid accumulation in atherosclerotic plaques to complications such as aneurysm formation and plaque disruption are poorly understood. BALB/c-Apoe−/− mice bearing a null mutation in the Npc1 gene display prominent medial erosion and atherothrombosis, whereas their macrophages accumulate free cholesterol in late endosomes and show increased cathepsin K (Ctsk) expression. We now show increased cathepsin K immunostaining and increased cysteinyl proteinase activity using near infrared fluorescence imaging over proximal aortas of Apoe−/−, Npc1−/− mice. In mechanistic studies, cholesterol loading of macrophage plasma membranes (cyclodextrin–cholesterol) or endosomal system (AcLDL+U18666A or Npc1 null mutation) activated Toll-like receptor (TLR) signaling, leading to sustained phosphorylation of p38 mitogen-activated protein kinase and induction of p38 targets, including Ctsk, S100a8, Mmp8, and Mmp14. Studies in macrophages from knockout mice showed major roles for TLR4, following plasma membrane cholesterol loading, and for TLR3, after late endosomal loading. TLR signaling via p38 led to phosphorylation and activation of the transcription factor Microphthalmia transcription factor, acting at E-box elements in the Ctsk promoter. These studies suggest that free cholesterol enrichment of either plasma or endosomal membranes in macrophages leads to activation of signaling via various TLRs, prolonged p38 mitogen-activated protein kinase activation, and induction of Mmps, Ctsk, and S100a8, potentially contributing to plaque complications.

Dopamine toxicity in neuroblastoma cells: role of glutathione depletion by L-BSO and apoptosis

Dopamine (DA), while an essential neurotransmitter, is also a known neurotoxin that potentially plays an etiologic role in several neurodegenerative diseases. DA metabolism and oxidation readily produce reactive oxygen species (ROS) and DA can also be oxidized to a reactive quinone via spontaneous, enzyme-catalyzed or metal-enhanced reactions. A number of these reactions are cytotoxic, yet the precise mechanisms by which DA leads to cell death remain unknown. In this study, the neuroblastoma cell line, SK-N-SH, was utilized to examine DA toxicity under varying oxidant states. Cells pretreated with the glutathione (GSH)-depleting compound, L-buthionine sulfoximine (L-BSO), exhibited enhanced sensitivity to DA compared to controls (non-GSH-depleted cells). Furthermore, in cells pretreated with L-BSO, the addition of ascorbate (250 microM) afforded significant protection against DA-induced toxicity, while pyruvate (500 microM) had no protective effect. To further characterize the possibility that DA is associated with oxidative stress, additional studies were carried out with manganese (30 microM) as a pro-oxidant. Manganese and DA (200 microM), although not cytotoxic when individually administered to SK-N-SH cells, had a synergistic action on cytotoxicity. Finally, morphological and molecular markers of programmed cell death (apoptosis) were observed in cells treated with DA and L-BSO. These markers included membrane blebbing and internucleosomal DNA fragmentation. These results suggest that DA toxicity is tightly linked to intracellular oxidant/antioxidant levels, and that environmental factors, such as excessive Mn exposure, may modulate cellular sensitivity to DA.

Size- and charge-dependent non-specific uptake of PEGylated nanoparticles by macrophages

The assessment of macrophage response to nanoparticles is a central component in the evaluation of new nanoparticle designs for future in vivo application. This work investigates which feature, nanoparticle size or charge, is more predictive of non-specific uptake of nanoparticles by macrophages. This was investigated by synthesizing a library of polymer-coated iron oxide micelles, spanning a range of 30–100 nm in diameter and −23 mV to +9 mV, and measuring internalization into macrophages in vitro. Nanoparticle size and charge both contributed towards non-specific uptake, but within the ranges investigated, size appears to be a more dominant predictor of uptake. Based on these results, a protease-responsive nanoparticle was synthesized, displaying a matrix metalloproteinase-9 (MMP-9)-cleavable polymeric corona. These nanoparticles are able to respond to MMP-9 activity through the shedding of 10–20 nm of hydrodynamic diameter. This MMP-9-triggered decrease in nanoparticle size also led to up to a six-fold decrease in nanoparticle internalization by macrophages and is observable by T2-weighted magnetic resonance imaging. These findings guide the design of imaging or therapeutic nanoparticles for in vivo targeting of macrophage activity in pathologic states.

Nascent high density lipoproteins formed by ABCA1 resemble lipid rafts and are structurally organized by three apoA-I monomers

This report details the lipid composition of nascent HDL (nHDL) particles formed by the action of the ATP binding cassette transporter A1 (ABCA1) on apolipoprotein A-I (apoA-I). nHDL particles of different size (average diameters of ∼12, 10, 7.5, and <6 nm) and composition were purified by size-exclusion chromatography. Electron microscopy suggested that the nHDL were mostly spheroidal. The proportions of the principal nHDL lipids, free cholesterol, glycerophosphocholine, and sphingomyelin were similar to that of lipid rafts, suggesting that the lipid originated from a raft-like region of the cell. Smaller amounts of glucosylceramides, cholesteryl esters, and other glycerophospholipid classes were also present. The largest particles, ∼12 nm and 10 nm diameter, contained ∼43% free cholesterol, 2–3% cholesteryl ester, and three apoA-I molecules. Using chemical cross-linking chemistry combined with mass spectrometry, we found that three molecules of apoA-I in the ∼9–14 nm nHDL adopted a belt-like conformation. The smaller (7.5 nm diameter) spheroidal nHDL particles carried 30% free cholesterol and two molecules of apoA-I in a twisted, antiparallel, double-belt conformation. Overall, these new data offer fresh insights into the biogenesis and structural constraints involved in forming nascent HDL from ABCA1.

Structures of Discoidal High Density Lipoproteins A COMBINED COMPUTATIONAL-EXPERIMENTAL APPROACH

Conversion of discoidal phospholipid (PL)-rich high density lipoprotein (HDL) to spheroidal cholesteryl ester-rich HDL is a central step in reverse cholesterol transport. A detailed understanding of this process and the atheroprotective role of apolipoprotein A-I (apoA-I) requires knowledge of the structure and dynamics of these various particles. This study, combining computation with experimentation, illuminates structural features of apoA-I allowing it to incorporate varying amounts of PL. Molecular dynamics simulated annealing of PL-rich HDL models containing unesterified cholesterol results in double belt structures with the same general saddle-shaped conformation of both our previous molecular dynamics simulations at 310 K and the x-ray structure of lipid-free apoA-I. Conversion from a discoidal to a saddle-shaped particle involves loss of helicity and formation of loops in opposing antiparallel parts of the double belt. During surface expansion caused by the temperature-jump step, the curved palmitoyloleoylphosphatidylcholine bilayer surfaces approach planarity. Relaxation back into saddle-shaped structures after cool down and equilibration further supports the saddle-shaped particle model. Our kinetic analyses of reconstituted particles demonstrate that PL-rich particles exist in discrete sizes corresponding to local energetic minima. Agreement of experimental and computational determinations of particle size/shape and apoA-I helicity provide additional support for the saddle-shaped particle model. Truncation experiments combined with simulations suggest that the N-terminal proline-rich domain of apoA-I influences the stability of PL-rich HDL particles. We propose that apoA-I incorporates increasing PL in the form of minimal surface bilayers through the incremental unwinding of an initially twisted saddle-shaped apoA-I double belt structure.

Membrane damage in leukemic cells induced by ether and ester lipids: An electron microscopic study

This paper reports a scanning electron microscopy (SEM) study of different leukemic cell lines exposed to 1-octadecyl-2-methyl-rac-glycero-3-phosphocholine (ET-18-OMe). This is an ether lipid analog of platelet activating factor (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine) which inhibits neoplastic cell growth in vitro and in vivo and is believed to exert its action through an interaction with the plasma membrane. In this paper evidence of the morphological alteration of leukemic cell membranes due to the exposure to varying concentrations of ET-18-OMe in vitro and in vivo is presented. This membrane damage consists of formation of blebs and holes, and the severity of these two phenomena correlate with the degree of cell viability loss. These alterations were analyzed in comparison to those induced by the known and structurally related permeabilizing agent, lysophosphatidylcholine, an ester lipid.

Effects of cellular cholesterol loading on macrophage foam cell lysosome acidification.

Macrophages incubated with mildly oxidized low density lipoprotein (OxLDL), aggregated low density lipoprotein (AggLDL), or cholesteryl ester-rich lipid dispersions (DISPs) accumulate cholesterol in lysosomes followed by an inhibition of lysosomal cholesteryl ester (CE) hydrolysis. The variety of cholesterol-containing particles producing inhibition of hydrolysis suggests that inhibition may relate to general changes in lysosomes. Lysosome pH is a key mediator of activity and thus is a potential mechanism for lipid-induced inhibition. We investigated the effects of cholesterol accumulation on THP-1 macrophage lysosome pH. Treatment with OxLDL, AggLDL, and DISPs resulted in inhibition of the lysosomes ability to maintain an active pH and concomitant decreases in CE hydrolysis. Consistent with an overall disruption of lysosome function, exposure to OxLDL or AggLDL reduced lysosomal apolipoprotein B degradation. The lysosomal cholesterol sequestration and inactivation are not observed in cholesterol-equivalent cells loaded using acetylated low density lipoprotein (AcLDL). However, AcLDL-derived cholesterol in the presence of progesterone (to block cholesterol egression from lysosomes) inhibited lysosome acidification. Lysosome inhibition was not attributable to a decrease in the overall levels of vacuolar ATPase. However, augmentation of membrane cholesterol in isolated lysosomes inhibited vacuolar ATPase-dependent pumping of H+ ions into lysosomes. These data indicate that lysosomal cholesterol accumulation alters lysosomes in ways that could exacerbate foam cell formation and influence atherosclerotic lesion development.