Free cholesterol-induced cytotoxicity a possible contributing factor to macrophage foam cell necrosis in advanced atherosclerotic lesions. Trends Cardiovasc Med. 1997;7:256-263. [PMID: 21235894 DOI: 10.1016/s1050-1738(97)00086-8]

Self-assembling biomaterials: liquid crystal phases of cholesteryl oligo(L-lactic acid) and their interactions with cells

We report here on the synthesis and characterization of a series of self-assembling biomaterials with molecular features designed to interact with cells and scaffolds for tissue regeneration. The molecules of these materials contain cholesteryl moieties, which have universal affinity for cell membranes, and short chains of lactic acid, a common component of biodegradable tissue engineering matrices. The materials were synthesized in good yields with low polydispersities in the range of 1.05-1.15, and their characterization was carried out by small-angle x-ray diffraction, transmission electron microscopy, electron diffraction, differential scanning calorimetry, and atomic force microscopy. These molecular materials form layered structures that can be described as smectic phases and can also order into single-crystal stacks with an orthorhombic unit cell. Their layer spacings range from 58 to 99 A, corresponding to bilayers of oligomers with an average of 10 and 37 lactic acid residues, respectively. The self-organized layered structures were found to promote improved fibroblast adhesion and spreading, although the specific mechanism for this observed response remains unknown. The ability of self-assembling materials to present ordered and periodic bulk structures to cells could be a useful strategy in tissue engineering.

Cholesterol and phospholipid metabolism in macrophages

Cholesterol-loaded macrophages are present at all stages of atherogenesis, and recent in vivo data indicate that these cells play important roles in both early lesion development and late lesion complications. To understand how these cells promote atherogenesis, it is critical that we understand how lesional macrophages interact with subendothelial lipoproteins, the consequences of this interaction, and the impact of subsequent intracellular metabolic events. In the arterial wall, macrophages likely interact with both soluble and matrix-retained lipoproteins, and a new challenge is to understand how certain consequences of these two processes might differ. Initially, the major intracellular metabolic route of the lipoprotein-derived cholesterol is esterification to fatty acids, but macrophages in advanced atherosclerotic lesions progressively accumulate large amounts of unesterified, or free, cholesterol (FC). In cultured macrophages, excess FC accumulation stimulates phospholipid biosynthesis, which is an adaptive response to protect the macrophage from FC-induced cytotoxicity. This phospholipid response eventually decreases with continued FC loading, leading to a series of cellular death reactions involving both death receptor-induced signaling and mitochondrial dysfunction. Because macrophage death in advanced lesions is thought to promote plaque instability, these intracellular processes involving cholesterol, phospholipid, and death pathways may play a critical role in the acute clinical manifestations of advanced atherosclerotic lesions.

Macrophages deficient in CTP:Phosphocholine cytidylyltransferase-alpha are viable under normal culture conditions but are highly susceptible to free cholesterol-induced death. Molecular genetic evidence that the induction of phosphatidylcholine biosynthesis in free cholesterol-loaded macrophages is an adaptive response

Macrophages in atherosclerotic lesions accumulate excess free cholesterol (FC) and phospholipid. Because excess FC is toxic to macrophages, these observations may have relevance to macrophage death and necrosis in atheromata. Previous work by us showed that at early stages of FC loading, when macrophages are still healthy, there is activation of the phosphatidylcholine (PC) biosynthetic enzyme, CTP:phosphocholine cytidylyltransferase (CT), and accumulation of PC mass. We hypothesized that this is an adaptive response, albeit transient, that prevents the FC:PC ratio from reaching a toxic level. To test this hypothesis directly, we created mice with macrophage-targeted disruption of the major CT gene, CTalpha, using the Cre-lox system. Surprisingly, the number of peritoneal macrophages harvested from CTalpha-deficient mice and their overall health under normal culture conditions appeared normal. Moreover, CT activity and PC biosynthesis and in vitro CT activity were decreased by 70-90% but were not absent. As a likely explanation of this residual activity, we showed that CTbeta2, a form of CT that arises from another gene, is induced in CTalpha-deficient macrophages. To test our hypothesis that increased PC biosynthesis is an adaptive response to FC loading, the viability of wild-type versus CTalpha-deficient macrophages under control and FC-loading conditions was compared. After 5 h of FC loading, death increased from 0.7% to only 2.0% in wild-type macrophages but from 0. 9% to 29.5% in CTalpha-deficient macrophages. These data offer the first molecular genetic evidence that activation of CTalpha and induction of PC biosynthesis in FC-loaded macrophages is an adaptive response. Furthermore, the data reveal that CTbeta2 in macrophages is induced in the absence of CTalpha and that a low level of residual CT activity, presumably due to CTbeta2, is enough to keep the cells viable in the peritoneum in vivo and under normal culture conditions.

Free cholesterol loading of macrophages induces apoptosis involving the fas pathway

Macrophage death is an important feature of atherosclerosis, but the cellular mechanism for this process is largely unknown. There is increasing interest in cellular free cholesterol (FC) excess as an inducer of lesional macrophage death because macrophages accumulate large amounts of FC in vivo, and FC loading of macrophages in culture causes cell death. In this study, a cell culture model was used to explore the cellular mechanisms involved in the initial stages of FC-induced macrophage death. After 9 h of FC loading, some of the macrophages exhibited externalization of phosphatidylserine and DNA fragmentation, indicative of an apoptotic mechanism. Incubation of the cells with Z-DEVD-fluoromethylketone blocked these events, indicating dependence upon effector caspases. Macrophages from mice with mutations in either Fas or Fas ligand (FasL) demonstrated substantial resistance to FC-induced apoptosis, and FC-induced death in wild-type macrophages was blocked by an anti-FasL antibody. FC loading had no effect on the expression of cell-surface Fas but caused a small yet reproducible increase in cell-surface FasL. To determine the physiological significance of this finding, unloaded and FC-loaded Fas-deficient macrophages, which can only present FasL, were compared for their ability to induce apoptosis in secondarily added Fas-bearing macrophages. The FC-loaded macrophages were much more potent inducers of apoptosis than the unloaded macrophages, and this effect was almost completely blocked by an inhibitory anti-FasL antibody. In summary, during the early stages of FC loading of macrophages, a fraction of cells exhibited biochemical changes that are indicative of apoptosis. An important part of this event is FC-induced activation of FasL that leads to Fas-mediated apoptosis. In light of recent in vivo findings that show that apoptotic macrophages in atherosclerotic lesions express both Fas and FasL, we present a cellular model of Fas-mediated death in lesional foam cells.

Absence of ACAT-1 attenuates atherosclerosis but causes dry eye and cutaneous xanthomatosis in mice with congenital hyperlipidemia

Acyl-CoA:cholesterol acyltransferase (ACAT) catalyzes esterification of cellular cholesterol. To investigate the role of ACAT-1 in atherosclerosis, we have generated ACAT-1 null (ACAT-1-/-) mice. ACAT activities were present in the liver and intestine but were completely absent in adrenal, testes, ovaries, and peritoneal macrophages in our ACAT-1-/- mice. The ACAT-1-/- mice had decreased openings of the eyes because of atrophy of the meibomian glands, a modified form of sebaceous glands normally expressing high ACAT activities. This phenotype is similar to dry eye syndrome in humans. To determine the role of ACAT-1 in atherogenesis, we crossed the ACAT-1-/- mice with mice lacking apolipoprotein (apo) E or the low density lipoprotein receptor (LDLR), hyperlipidemic models susceptible to atherosclerosis. High fat feeding resulted in extensive cutaneous xanthomatosis with loss of hair in both ACAT-1-/-:apo E-/- and ACAT-1-/-:LDLR-/- mice. Free cholesterol content was significantly increased in their skin. Aortic fatty streak lesion size as well as cholesteryl ester content were moderately reduced in both double mutant mice compared with their respective controls. These results indicate that the local inhibition of ACAT activity in tissue macrophages is protective against cholesteryl ester accumulation but causes cutaneous xanthomatosis in mice that lack apo E or LDLR.

Interactions between metabolism and intracellular distribution of cholesterol and sphingomyelin

There is ample evidence from experimental models and human metabolic disorders indicating that cholesterol and sphingomyelin (SM) levels are coordinately regulated. Generally it has been observed that altering the cellular content of sphingomyelin or cholesterol results in corresponding changes in mass and/or synthesis of the other lipid. In the case of cholesterol synthesis and trafficking, SM regulates the capacity of membranes to absorb cholesterol and thereby controls sterol flux between the plasma membrane and regulatory pathways in the endoplasmic reticulum. This relationship exemplifies the importance of cholesterol/sphingolipid-rich domains in cholesterol homeostasis, as well as other aspects of cell signaling and transport. Evidence for regulation of sphingomyelin metabolism by cholesterol is less convincing and dependent on the model system under study. Sphingomyelin biosynthetic rates are not dramatically affected by alterations in cholesterol balance suggesting that sphingomyelin or its metabolites serve other indispensable functions in the cell. A notable exception is the robust and specific regulation of both SM and cholesterol synthesis by 25-hydroxycholesterol. This finding is reviewed in the context of the role of oxysterol binding protein and its putative role in cholesterol and SM trafficking between the plasma membrane and Golgi apparatus.

Massive xanthomatosis and altered composition of atherosclerotic lesions in hyperlipidemic mice lacking acyl CoA:cholesterol acyltransferase 1

Inhibitors of acyl CoA:cholesterol acyltransferase (ACAT) have attracted considerable interest as a potential treatment for atherosclerosis. Currently available inhibitors probably act nonselectively against the two known ACATs. One of these enzymes, ACAT1, is highly expressed in macrophages in atherosclerotic lesions, where it contributes to foam-cell formation. In this study, we examined the effects of selective ACAT1 deficiency in two mouse models of atherosclerosis. In the setting of severe hypercholesterolemia caused by deficiency in apoE or the LDL receptor (LDLR), total ACAT1 deficiency led to marked alterations in cholesterol homeostasis and extensive deposition of unesterified cholesterol in the skin and brain. Bone marrow transplantation experiments demonstrated that ACAT1 deficiency in macrophages was sufficient to cause dermal xanthomas in hyperlipidemic LDLR-deficient mice. ACAT1 deficiency did not prevent the development of atherosclerotic lesions in either apoE-deficient or LDLR-deficient mice, despite causing relatively lower serum cholesterol levels. However, the lesions in ACAT1-deficient mice were atypical in composition, with reduced amounts of neutral lipids and a paucity of macrophages in advanced lesions. Although the latter findings may be associated with increased lesion stability, the marked alterations in cholesterol homeostasis indicate that selectively inhibiting ACAT1 in the setting of severe hyperlipidemia may have detrimental consequences.

Modified LDL - trigger of atherosclerosis and inflammation in the arterial intima

Atherosclerosis is characterized by chronic inflammation of an injured intima. The pathological processes are initiated by accumulation of morphologically distinct, modified forms of LDL, and followed by cellular infiltration and foam cell formation. Activated intimal cells secrete enzymes and agents capable of modifying LDL, and the modified lipids of LDL, in turn, are able to activate intimal cells and to trigger various inflammatory signals. These processes can initiate and maintain a vicious circle in the intima and lead to lesion progression. In this review, we focus on the LDL modifications relevant to the initial lipid accumulation and discuss their pro-inflammatory effects.

Macrophage-targeted CTP:phosphocholine cytidylyltransferase (1-314) transgenic mice

CTP:phosphocholine cytidylyltransferase (CT) is a rate-limiting and complexly regulated enzyme in phosphatidylcholine (PC) biosynthesis and is important in the adaptation of macrophages to cholesterol loading. The goal of the present study was to use transgenesis to study the CT reaction in differentiated macrophages in vivo. We successfully created macrophage-targeted transgenic mice that overexpress a truncated form of CT, called CT-314. Sonicated homogenates of peritoneal macrophages overexpressing CT-314 protein demonstrated a two-fold increase in CT activity in vitro compared with homogenates from nontransgenic macrophages. CT-314 macrophages, however, demonstrated no increase in CT activity or PC biosynthesis in vivo. This finding could not be explained simply by intracellular mistargeting of CT-314, by the inability of CT-314 to associate with cellular membranes, or by substrate limitation. To further probe the mechanism, an in vitro assay using intact nuclei was developed in an attempt to preserve interactions between CT, which is primarily a nuclear enzyme in macrophages, and other nuclear molecules. This intact-nuclei assay faithfully reproduced the situation observed in living macrophages, namely, no significant increase in CT activity despite increased CT-314 protein. In contrast, CT activity in sonicated nuclei from CT-314 macrophages was substantially higher than that from nontransgenic macrophages. Thus, a sonication-sensitive interaction between excess CT and one or more nuclear molecules may be responsible for the limitation of CT activity in CT-314 macrophages. These data represent the first report of a CT transgenic animal and the first study of a differentiated cell type with excess CT.

Cholesterol distribution in living cells: fluorescence imaging using dehydroergosterol as a fluorescent cholesterol analog

Cholesterol is an important constituent of most mammalian cell membranes and its concentration in various cellular membranes is tightly regulated. Although there is much information about cholesterol distribution and trafficking in cells, it is primarily derived from indirect measurements, and the results obtained using different approaches are often conflicting. A cholesterol analog that faithfully mimics the properties of cholesterol and can be followed in living cells would thus be very useful. In this study, we report the fluorescence imaging of such an analog, dehydroergosterol (DHE), in living cells. DHE differs from cholesterol in having three additional double bonds and an extra methyl group. In model systems, DHE closely mimics the behavior of native cholesterol. Using triple-labeling studies, we show that DHE colocalizes extensively with endocytosed transferrin, an endocytic recycling compartment marker, and with a marker for the trans-Golgi network, Tac-TGN38. This distribution of DHE is qualitatively similar to that observed when cells are labeled with the fluorescent cholesterol-binding polyene antibiotic, filipin, although there are differences in apparent proportions of DHE and filipin that are localized at the plasma membrane. Another cholesterol derivative, 25-NBD-cholesterol, has a structure that is compromised by the presence of a bulky NBD group and does not distribute to the same organelles as DHE or filipin. In addition, we show in this manuscript that kinetic processes can be followed in living cells by monitoring recovery of DHE fluorescence in a photobleached region over time. Our observations provide evidence for the presence of a large intracellular cholesterol pool in the endocytic recycling compartment and the trans-Golgi network that might play important roles in the trafficking of lipids, lipid-anchored proteins, and transmembrane proteins that preferentially partition into cholesterol-enriched membrane domains. In addition, this intracellular cholesterol pool might be involved in the maintenance of cellular cholesterol homeostasis.