New programs for gene- and cell-based therapies at NHLBI.

The development of new and novel clinical therapies depends on our ability to translate basic research findings into meaningful clinical applications. These "translational studies" are expensive and complex but necessary if we are to develop new treatments that prevent or treat heart, lung, and blood diseases. This study describes newly created programs at NHLBI (National Heart, Lung, and Blood Institute), which support academic research efforts to translate gene- and cell-based therapies into viable clinical applications.

Apolipoprotein B stimulates formation of monocyte-macrophage surface-connected compartments and mediates uptake of low density lipoprotein-derived liposomes into these compartments.

Much of the cholesterol that accumulates in atherosclerotic plaques is found within monocyte-macrophages transforming these cells into "foam cells." Native low density lipoprotein (LDL) does not cause foam cell formation. Treatment of LDL with cholesterol esterase converts LDL into cholesterol-rich liposomes having >90% cholesterol in unesterified form. Similar cholesterol-rich liposomes are found in early developing atherosclerotic plaques surrounding foam cells. We now show that cholesterol-rich liposomes produced from cholesterol esterase-treated LDL can cause human monocyte-macrophage foam cell formation inducing a 3-5-fold increase in macrophage cholesterol content of which >60% is esterified. Although cytochalasin D inhibited LDL liposome-induced macrophage cholesteryl ester accumulation, LDL liposomes did not enter macrophages by phagocytosis. Rather, the LDL liposomes induced and entered surface-connected compartments within the macrophages, a unique endocytic pathway in these cells that we call patocytosis. LDL liposome apoB rather than LDL liposome lipid mediated LDL liposome uptake by macrophages. This was shown by the findings that: 1) protease treatment of the LDL liposomes prevented macrophage cholesterol accumulation; 2) liposomes prepared from LDL lipid extracts did not cause macrophage cholesterol accumulation; and 3) purified apoB induced and accumulated within macrophage surface-connected compartments. Although apoB mediated the macrophage uptake of LDL liposomes, this uptake did not occur through LDL, LDL receptor-related protein, or scavenger receptors. Also, LDL liposome uptake was not sensitive to treatment of macrophages with trypsin or heparinase. Cholesterol esterase-mediated transformation of LDL into cholesterol-rich liposomes is an LDL modification that: 1) stimulates uptake of LDL cholesterol by apoB-dependent endocytosis into surface-connected compartments, and 2) causes human monocyte-macrophage foam cell formation.

Accumulation of HDL apolipoproteins accompanies abnormal cholesterol accumulation in Schnyder's corneal dystrophy.

Schnyder's corneal dystrophy is an autosomal dominant disorder that results in clouding of the central cornea and premature development of peripheral arcus in the cornea. Previous studies showed that abnormal lipid accumulation is the basis for the corneal clouding. We examined whether apolipoproteins are involved in this disorder and characterized the lipid accumulation in the central portion of corneas removed from patients with Schnyder's dystrophy. Our findings show that cholesterol and phospholipid contents increased greater than 10-fold and 5-fold, respectively, in affected compared with normal corneas. In addition, the percentage of cholesterol that was unesterified (63% versus 50%) and the molar ratio of unesterified cholesterol to phospholipid (1.5 versus 0.5) were higher in affected compared with normal corneas. Large multilamellar vesicles and electron-dense granules (100 to 300 nm in diameter) as well as cholesterol crystals accumulated in the extracellular matrix of affected corneas. Immunohistochemical analysis showed that apolipoprotein constituents of HDL (apoA-I, apoA-II, and apoE), but not apoB, a marker of LDL, accumulated in the affected cornea. Western blot analysis confirmed the increased amounts of these HDL apolipoproteins in affected corneas and showed that the apparent molecular weights of the apolipoproteins were normal. Our findings show for the first time that HDL apolipoproteins accumulate in the corneas of patients with Schnyder's corneal dystrophy. Thus, this disorder influences the metabolism of HDL in the corneas of these patients.

Sequestration of acetylated LDL and cholesterol crystals by human monocyte-derived macrophages.

Monocyte-derived macrophages accumulate and process cholesterol in atherosclerotic lesions. Because of the importance of this process, we examined the interaction of cholesterol crystals and acetylated low density lipoprotein (AcLDL) with human monocyte-macrophages in a combined chemical and morphological study. These two forms of cholesterol induced extensive compartmentalization of the macrophage cytoplasm. Unexpectedly, the compartments maintained a physical connection to the extracellular space as demonstrated with ruthenium red staining. The compartments formed through invagination of the top surface of the macrophage plasma membrane. Some cholesterol crystals and AcLDL were sequestered within these surface-connected compartments for up to five days in the case of the crystals and for one day in the case of AcLDL. Pulse-chase studies of fractionated macrophages indicated that [3H]cholesterol redistributed from the surface-connected compartments into lysosomes (where the cholesterol remained unesterified) and into lipid droplets (where the cholesterol was stored as cholesteryl ester). Intracellular uptake and esterification of cholesterol was blocked by cytochalasin D. However, once cholesterol was sequestered in the surface-connected compartments, subsequent esterification of the cholesterol could not be inhibited by cytochalasin D. Apolipoprotein E was localized within the surface-connected compartments by immunogold labeling suggesting a possible function for this protein in the processing of lipid taken up through the sequestration pathway. Removal of microcrystalline cholesterol from the medium resulted in release of most of the accumulated cholesterol microcrystals from the macrophages, as well as disappearance of the surface-connected compartments. Thus, sequestration is a novel endocytic mechanism in which endocytic compartments remain connected to the extracellular space. This differs from phagocytosis where endocytic vacuoles rapidly pinch off from the plasma membrane. Sequestration provides a means for macrophages to remove substances from the extracellular space and later release them.

Cholesterol efflux from human monocyte-derived macrophages in the presence of LpA-I:A-II.

Previous epidemiological studies have suggested that the LpA-I subfraction of HDL is more protective than the LpA-I:A-II subfraction against the development of cardiovascular disease. A possible basis for a specific anti-atherogenic function of LpA-I emerged from studies of cholesterol efflux from cultured mouse adipocytes. LpA-I efficiently removed excess cholesterol from the mouse adipocytes, while LpA-I:A-II was ineffective. On the other hand, LpA-I:A-II was able to stimulate cholesterol efflux from a number of other cell types including rodent macrophages. Because of previously reported differences in HDL stimulation of cholesterol clearance from macrophages of different origins, we determined whether LpA-I:A-II could induce cholesterol efflux from cultured human monocyte-macrophages. Our findings showed that LpA-I:A-II and HDL3 effectively stimulated cholesterol efflux from human monocyte-macrophages enriched with cholesterol by incubation with AcLDL. LpA-I:A-II also decreased by one-half the amount of cholesterol accumulated when macrophages were incubated with AcLDL and LpA-I:A-II together. Thus, it would appear that the differential anti-atherogenic effects of LpA-I:A-II and LpA-I do not derive from their effects on macrophage cholesterol efflux. Possibly these HDL subfractions differentially affect other biologic processes that modulate the development of cardiovascular disease.

Production of cholesterol-enriched nascent high density lipoproteins by human monocyte-derived macrophages is a mechanism that contributes to macrophage cholesterol efflux.

Atherosclerotic lesions have a lipid core containing crystals and liposomes enriched in unesterified cholesterol as well as numerous monocyte-macrophages enriched in cholesteryl ester. Sufficient amounts of plasma-derived high density lipoproteins (HDL) may not reach and efficiently remove the cholesterol deposited in lesion macrophages or in the lipid core of lesions. We examined the potential of human monocyte-macrophages to produce nascent HDL and to solubilize cholesterol derived from interaction of monocyte-macrophages with lipoprotein and non-lipoprotein sources of cholesterol. Monocyte-macrophages produced discoidal (25 +/- 6 nm long and 6 +/- 1 nm wide (mean +/- S.D.)) and vesicular (89 +/- 41 nm in diameter) lipoprotein particles following and during enrichment of macrophages with cholesterol from acetylated low density lipoprotein (AcLDL) or cholesterol crystals. During cholesterol enrichment, discoidal particles progressively accumulated in the medium for up to 6 days. In contrast, vesicles did not increase past 2 days of incubation. Both the discoidal and vesicular lipoprotein particles had a peak density of about 1.09-1.10 g/ml. The discoidal particles contained apolipoprotein E (apoE), whereas the vesicles contained a major protein constituent with a molecular mass of 22,000 daltons. The vesicles did not contain detectable apoE and the 22,000-dalton protein was not the 22,000-dalton thrombolytic fragment of apoE. Following cholesterol enrichment of macrophages with AcLDL or cholesterol crystals, macrophages excreted much of their accumulated cholesterol, even in the absence of exogenously added cholesterol acceptors. Most of this excreted cholesterol was recovered from the culture medium and was carried in the apoE discoidal particles that showed cholesterol enrichment up to a 2:1 unesterified cholesterol to phospholipid molar ratio. The findings suggest that sufficient production of these nascent HDL by macrophages within atherosclerotic lesions should facilitate removal of cellular and extracellular cholesterol, even in the absence of plasma-derived HDL.

Absence of triglyceride accumulation in lipoprotein lipase-deficient human monocyte-macrophages incubated with human very low density lipoprotein.

Lipoprotein lipase, a lipolytic enzyme essential for normal hydrolysis of triglycerides in very low density lipoprotein (VLDL) and chylomicrons, is found in several cell types, including macrophages. The role of lipoprotein lipase in mediating the uptake of normal VLDL triglycerides into human cultured monocyte-derived macrophages was studied using macrophage cells from a functionally lipoprotein lipase-deficient patient and macrophages of cells from a normal subject. After incubation with VLDL, massive accumulation of phase refractile (lipid) inclusions were noted by phase contrast microscopy within the normal, but not within the lipoprotein lipase-deficient, macrophages. Chemical determinations of intracellular lipid confirmed massive triglyceride accumulation within normal macrophages, but not in lipoprotein lipase-deficient macrophages. VLDL-derived cholesterol did not accumulate in either cell. These results confirm an additional role of lipoprotein lipase, that of mediating triglyceride accumulation into macrophages from normal human VLDL. Human monocyte-macrophages genetically deficient in a functional lipoprotein lipase will be useful to determine the role of lipoprotein lipase in macrophage accumulation of lipid from other forms of triglyceride-carrying lipoproteins, including hypertriglyceridemic VLDL, beta-VLDL, and chylomicrons.

Heterogeneity of cellular cholesteryl ester accumulation by human monocyte-derived macrophages.

We have studied cholesteryl ester accumulation in human monocyte-derived macrophages, which together with smooth muscle cells, represent the major cell types that accumulate cholesterol in atherosclerotic lesions. Monocyte-derived macrophages were incubated with either acetylated low density lipoprotein (AcLDL) or non-lipoprotein cholesterol and the question as to whether all of the cells, or specific cell subpopulations could accumulate cholesteryl ester was examined. We stained cholesteryl ester in monocyte-macrophages with the fluorescent probe filipin. Cholesteryl ester accumulated as lipid droplets that were widely dispersed in the cell cytoplasm. Interestingly, no more than 65% of monocyte-macrophages accumulated cholesteryl ester during the 1st day of incubation with non-lipoprotein cholesterol. By 2 days of incubation, greater than 90% of cells displayed cholesteryl ester deposition. The cholesteryl ester which accumulated during the 2nd day of incubation was derived from unesterified cholesterol that had accumulated during the 1st day of incubation. This finding was substantiated by the following: (1) chemical measurements showed that the total cholesterol content of monocyte-macrophages did not increase further after the 1st day of incubation, and (2) all monocyte-macrophages had accumulated fluorescent tagged cholesterol during the 1st day of incubation. In contrast to the results obtained with non-lipoprotein cholesterol, more than 90% of monocyte-macrophages incubated with AcLDL for 1 day accumulated cholesteryl ester in two experiments. However, less than 62% of monocyte-macrophages accumulated cholesteryl ester in two other experiments, thereby resembling results obtained with non-lipoprotein cholesterol. Again, the lack of cholesteryl ester accumulation with AcLDL was not due to a lack of uptake of AcLDL, as greater than 90% of monocyte-macrophages accumulated fluorescent tagged AcLDL. The observed heterogeneity in cholesterol esterification among human monocyte-macrophages suggests that functional subpopulations of these cells may exist with respect to cholesterol processing. However, heterogeneity in cholesteryl ester accumulation did not seem to correlate with expression of HLA-DR antigen, a marker of immunological activation of macrophages. Other sources of heterogeneity most likely result from inter-cellular variation at one or more levels of regulation of the cholesterol trafficking and esterification process.

Phospholipid loss in dying platelets.

The death of a cell results in a large amount of membrane lipid, predominantly phospholipids and cholesterol, that must be eliminated. In this study, we have examined what happens to phospholipids in dying rat platelets. Rat platelets were incubated for up to three days following their activation with thrombin. Platelet death occurred during the first day of incubation. This was indicated by a complete loss of platelet lactate dehydrogenase into the incubation medium. The platelets progressively lost over one-half of their phospholipid content during the three days of incubation. Cholesterol and sphingomyelin (the phospholipid with the highest affinity for cholesterol) were not lost during the same period. Our findings suggest that significant degradation of cellular non-sphingomyelin phospholipid can be triggered by cell death. The preservation of sphingomyelin in dying platelets, may be an adaptive response to maintain cholesterol in a solubilized state within dying cells.

Familial apolipoprotein E deficiency and type III hyperlipoproteinemia due to a premature stop codon in the apolipoprotein E gene.

A kindred with apolipoprotein E deficiency and a truncated lower molecular weight apoE mutant, designated apoE-3Washington, has been identified. Gel electrophoresis demonstrated complete absence of the normal apoE isoproteins and the presence of a small quantity of a lower molecular weight apoE. Plasma apoE levels in the proband were approximately 4% of normal. This marked deficiency of apoE resulted in delayed uptake of chylomicron and very low density lipoprotein (VLDL) remnants by the liver, elevated plasma cholesterol levels, mild hypertriglyceridemia, and the development of type III hyperlipoproteinemia. Sequence analysis of the patient's apoE gene revealed a single nucleotide substitution of an A for a G, which converted amino acid 210 of the mature protein, tryptophan (TGG), to a premature chain termination codon (TAG), thus leading to the synthesis of a truncated E apolipoprotein of 209 amino acids with a molecular mass of 23.88 kDa. Northern blot analysis of differentiated monocyte-derived macrophages demonstrated a mutant mRNA indistinguishable in size from normal apoE mRNA. The nucleotide substitution also resulted in the formation of a new restriction site for Mae I. Using this enzyme we were able to establish that the proband is a homozygote and that her two offsprings are heterozygous for the epsilon-3Washington allele. These data demonstrate that the striking deficiency of apoE-3Washington results in a moderate form of type III hyperlipoproteinemia. The clinical presentation also suggests a dispensable role of apoE in the nervous system and in immunoregulation.

Hydrolysis of cholesteryl ester in low density lipoprotein converts this lipoprotein to a liposome.

Previously, we isolated and characterized unique liposomal-like, cholesterol-rich lipid particles that accumulate in human atherosclerotic lesions. Human plasma low density lipoprotein (LDL) has a molar ratio of total cholesterol to phospholipid (3:1) similar to that of this lesion cholesterol-rich lipid particle. However, LDL is enriched in cholesteryl ester while the lesion lipid particle is enriched in unesterified cholesterol. To examine a possible precursor-product relationship between LDL and the lesion lipid particle, we hydrolyzed the cholesteryl ester core of LDL with cholesterol esterase. Cholesteryl ester hydrolysis occurred only after LDL was treated with trypsin. Trypsin pretreatment was not required for cholesteryl ester hydrolysis of LDL oxidized with copper, a treatment that also degrades apolipoprotein B, the major protein moiety in LDL. In contrast to greater than 90% hydrolysis of cholesteryl ester in trypsin-cholesterol esterase-treated or copper-oxidized LDL, there was only 18% hydrolysis of cholesteryl ester in similarly treated high density lipoprotein. With a limited 10-min hydrolysis of LDL cholesteryl ester, LDL-sized particles and newly formed larger flattened films or discs were present. With complete hydrolysis of LDL cholesteryl ester, LDL particles converted to complex multilamellar, liposomal-like, structures with sizes approximately five times larger than native LDL. These liposomal-like particles derived from LDL were chemically and structurally similar to unesterified cholesterol-rich lipid particles that accumulate in atherosclerotic lesions.

Identification of two separate allelic mutations in the lipoprotein lipase gene of a patient with the familial hyperchylomicronemia syndrome.

The molecular defects resulting in a deficiency of lipoprotein lipase activity in a patient with the familial hyperchylomicronemia syndrome have been identified. Increased lipoprotein lipase mass but undetectable lipoprotein lipase activity in the patient's post-heparin plasma indicate the presence of an inactive enzyme. No major gene rearrangements were identified by Southern blot analysis of the patient's lipoprotein lipase gene and Northern blot hybridization revealed an lipoprotein lipase mRNA of normal size. Sequence analysis of polymerase chain reaction-amplified lipoprotein lipase cDNA identified two separate allelic mutations. A T to C transition at nucleotide 836 results in the substitution of Ile194, located near the putative interfacial recognition site of lipoprotein lipase, to a Thr. A G to A mutation at base 983 leads to the substitution of a His for Arg243 and the loss of a HhaI restriction enzyme site. Arg243 is near His241, which has been postulated to be part of the catalytic triad of lipoprotein lipase. Direct sequencing of amplified cDNA and digestion with HhaI established that the proband is a compound heterozygote for each base substitution. Transient expression of each of the mutant lipoprotein lipase cDNAs in human embryonal kidney-293 cells resulted in the synthesis of enzymically inactive proteins, establishing the functional significance of the mutations. We conclude that the Ile194 to Thr194 and Arg243 to His243 substitutions occur in lipoprotein lipase regions essential for normal enzyme activity and each mutation results in the expression of a nonfunctional enzyme leading to the hyperchylomicronemia syndrome manifested in the proband.

Lipoprotein lipaseBethesda: a single amino acid substitution (Ala-176----Thr) leads to abnormal heparin binding and loss of enzymic activity.

The molecular defect that leads to a deficiency of lipoprotein lipase (LPL) activity in the proband from a Bethesda kindred has been identified. The pre- and post-heparin plasma LPL mass in the proband was elevated when compared to controls; however, there was no detectable LPL activity, indicating the presence of a defective enzyme (termed LPLBethesda). Analysis of the patient's post-heparin plasma by heparin-Sepharose affinity chromatography demonstrated that the mutant LPL had an altered affinity for heparin. Southern blot hybridization of the gene for LPLBethesda revealed no major rearrangements. Northern blot analysis of LPLBethesda mRNA from patient monocyte-derived macrophages revealed normal-sized mRNAs (3.4 and 3.7 kilobases) as well as normal cellular mRNA levels when compared to control macrophages. Sequence analysis of polymerase chain reaction-amplified LPL cDNA revealed a G----A substitution at position 781 of the normal LPL gene that resulted in the substitution of an alanine for a threonine at residue 176 and the loss of an SfaNI site present in the normal LPL gene. Amplification of cDNA by the PCR followed by digestion with SfaNI established that the patient was a true homozygote for the mutation. Expression of LPL cDNA in COS-7 cells resulted in the synthesis of a nonfunctional LPL enzyme establishing that the Ala----Thr substitution was the mutation responsible for the inactive LPL. The identification of this mutation in the LPL gene defines a region of the LPL enzyme, at Ala-176, that is essential for normal heparin-binding and catalytic activity. We propose that an amino acid substitution in this critical region of LPLBethesda results in the synthesis of a nonfunctional enzyme that leads to the chylomicronemia syndrome expressed in this proband.

Characterization of two unique cholesterol-rich lipid particles isolated from human atherosclerotic lesions.

The authors' laboratory, using histochemical methods, previously identified two types of cholesterol-containing lipid particles in the extracellular spaces of human atherosclerotic lesions, one particle enriched in esterified cholesterol and the other particle enriched in unesterified cholesterol. The authors isolated and characterized these lipid particles. The esterified cholesterol-rich lipid particle was a small lipid droplet and differed from intracellular lipid droplets found in foam cells with respect to size and chemical composition. It had an esterified cholesterol core surrounded by a phospholipid-unesterified cholesterol monolayer. Some aqueous spaces were seen within the particle core. Unesterified cholesterol-rich lipid particles were multilamellated, solid structures and vesicles comprised of single or multiple lamellas. The esterified cholesterol-rich particle had a density less than 1.01 g/ml, whereas the unesterified cholesterol-rich particle had a density between 1.03 and 1.05 g/ml. Both particles were similar in size (90% of both particles ranged in size between 40 to 200 nm in diameter) and had an unesterified cholesterol-to-phospholipid molar ratio of 2.5:1. The predominant phospholipid in both particles was sphingomyelin. The fatty acyl compositions of cholesteryl ester and phospholipid also were similar in both particles. Palmitate, oleate, and linoleate were the major fatty acids in the cholesteryl ester fraction, whereas palmitate, stearate, oleate, and linoleate were predominant in the phospholipid fraction. The origins and the role of these two unusual lipid particles in vessel wall cholesterol metabolism remain to be determined.

Biochemical characterization of isolated cholesterol-phospholipid particles continuously released from rat and human platelets after activation.

Platelets are a source of lipid that facilitates the coagulation process and that potentially accumulates in cells within atherosclerotic lesions. With this in mind, we have examined the release of lipid-containing particles from activated rat and human platelets. When washed platelets were activated with thrombin and incubated, cholesterol and phospholipid were continuously released for a period up to 2 hours. The amount of cholesterol released was approximately 20% of the platelet cholesterol content. Cholesterol release was also stimulated by strong platelet agonists such as collagen and the calcium ionophore A23187 but not by the weak agonist ADP. The release of cholesterol did not simply result from lysis of platelets since lactate dehydrogenase and cholesterol release could be dissociated. Colchicine substantially inhibited release of cholesterol but did not substantially inhibit release of lactate dehydrogenase. Cholesterol-phospholipid particles were isolated from platelet-release supernatants by centrifugation, microfiltration, and gel filtration chromatography. The particles, which eluted in the void volume, were composed of 57% protein, 32% phospholipids, 8% cholesterol, and 3% triglyceride. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of particle-associated protein showed a major protein component of Mr 42,000, presumably actin. The density of the particles in a sucrose density gradient was 1.16 g/ml. The cholesterol to phospholipid molar ratio of isolated particles from rat and human platelets was about 0.6, a value typical of plasma membranes, whereas the cholesterol to phospholipid molar ratio in particles released from platelets of rats fed a high-cholesterol diet increased to 1.0. Electron microscopic analysis of the particles showed them to be spherical or irregular in shape with sizes ranging from 50 to 550 nm and with projections that extended from their surfaces. We suggest that these cholesterol-phospholipid particles released from platelets may represent a mechanism by which the membrane becomes dispersed following platelet activation.

Unesterified cholesterol-rich lipid particles in atherosclerotic lesions of human and rabbit aortas.

Unesterified cholesterol-rich lipid particles were isolated from human and cholesterol-fed rabbit aortas. These particles have been previously reported to constitute the initial lipid deposition in atherosclerotic lesion development. Purification of the particles was accomplished with microfiltration, gel filtration chromatography, and density gradient centrifugation. Particles from both human and rabbit aortas had a density of between 1.02 g/ml and 1.08 g/ml with a peak at d = 1.036 g/ml. These particles had a high molar ratio of unesterified cholesterol to phospholipid (2.4:1 in rabbit, 2.6:1 in human) and a high percentage of their cholesterol in an unesterified form (82% in rabbit, 76% in human). The particles had diameters between 700 and 3000 A and showed unilamellar and multilamellar structures. Freeze-fractured particles had smooth fracture faces and sometimes contained a smooth-surfaced core. Upon incubation with filipin, particles showed typical filipin-sterol complexes, demonstrating the presence of unesterified cholesterol. The particles we have isolated may constitute an early pathologic form of accumulated cholesterol in developing lesions and may represent a degradation product of infiltrated plasma low-density lipoprotein.

Cultured bovine aortic endothelial cells show increased histamine metabolism when exposed to oscillatory shear stress.

Oscillatory shear stress applied to the lining of blood vessels causes endothelial cell injury, one of the essential postulated prerequisites to the development of atherosclerosis. The purpose of this investigation was to study effects of shear stress on bovine aortic endothelial cells (BAEC), in vitro, for varying lengths of time (6 h, 12 h, 24 h) on BAEC histamine content (HC) and histidine decarboxylase activity (HD). Low intensity stress (1.6 dynes/cm2) as well as intermediate and high intensity shear stresses (3.5 dynes/cm2 and 7.6 dynes/cm2) resulted in an accelerated HD (281%) and elevated HC (144%). These data indicate that oscillatory shear stress produces increases in histamine metabolism.