ACAT Inhibition
http://www.100md.com
《动脉硬化血栓血管生物学》
From the Department of Pathology (S.F., D.E.D.), Department of Medicine, Division of Cardiovascular Medicine (S.F., M.F.L.), and Department of Pharmacology (M.F.L.), Vanderbilt University Medical Center, Nashville, Tenn.
Correspondence to Sergio Fazio, Vanderbilt University Medical Center, Division of Cardiovascular Medicine, 383 Preston Research Building, Nashville, TN 37232-6300. E-mail sergio.fazio@vanderbilt.edu
Ahallmark of the atherosclerotic plaque is the accumulation of lipid-laden foam cells that are derived from macrophages or smooth muscle cells (SMCs).1,2 Cholesterol from modified lipoproteins is taken up and either stored in intracellular lipid droplets or delivered to extracellular cholesterol acceptors. Cellular free cholesterol can be exported to nascent HDL and activate the reverse cholesterol transport system, but when it accumulates in large amounts it can induce toxic changes leading to cell death. Conversely, esterified cholesterol is a more inert form of cholesterol for storage, but is not available for efflux to extracellular cholesterol acceptors. Acyl-coenzyme A (CoA): cholesterol acyltransferase (ACAT) esterifies excess free cholesterol and regulates cellular cholesterol homeostasis. ACAT occurs in two isoforms, ACAT-1 and ACAT-2. Whereas liver and intestine express both ACAT-1 and ACAT-2, macrophages and SMCs express ACAT-1 only.3 Nonselective inhibition of ACAT has the potential to both lower plasma lipids and to reduce foam cell formation.3,4 However, the therapeutic potential of ACAT inhibitors is not without controversy, because data have been published suggesting beneficial effects on experimental atherosclerosis as well as negative effects on macrophage viability.5–7 Furthermore, elimination of macrophage ACAT-1 is associated with increased atherosclerotic lesion formation in murine models.8 In this issue of Arteriosclerosis, Thrombosis, and Vascular Biology, Rong et al point to SMCs as a possible explanation for the apparently contradictory effects of ACAT inhibition.9
See page 122
In this study, Rong et al confirm the role of ACAT-1 in the formation of SMC foam cells and show that SMCs are resistant to the toxic effects of ACAT inhibition and free cholesterol (Figure).9 SMCs that were treated with cholesterol cyclodextrin complexes produced foam cells with lipid droplets and elevated levels of cholesteryl esters. The ACAT inhibitor F-1394 was able to block the accumulation of lipid droplets. More importantly, the authors found a significant difference between SMC and macrophages with regard to cholesterol-induced toxicity. ACAT inhibition that caused cholesterol toxicity in cholesterol-loaded macrophages was apparently well tolerated cholesterol-loaded SMC. This finding provides a potential explanation for how ACAT inhibition can cause a decrease in the size of the macrophage foam-cell lesion and an apparent increase in the number of SMCs in the plaque.5
SMC Foam cells are resistant to cholesterol toxicity. Both macrophages and SMCs can assume a foam cell phenotype. Inhibition of ACAT, the enzyme that esterifies intracellular cholesterol, can have toxic consequences for macrophages in vitro. The resistance of SMC foam cells to free cholesterol toxicity may represent the basis for the mixed effects of ACAT inhibition on plaque burden.
These interesting results might have been partly influenced by the unique loading method, which redistributes large amounts of cholesterol to the plasma membrane. It is not known whether cholesterol that enters through the plasma membrane has the same toxic potential as cholesterol that enters by receptor-mediated uptake of modified lipoproteins or phagocytic uptake of cellular debris. As for the resistance of SMC foam cells to toxicity, it is possible that relative to macrophages SMC have less cholesterol routed to toxic pools or more efficient removal of cholesterol from these pools.
The exact nature of cytotoxic pools of cholesterol is unknown. Although these pools are associated with both plasma membrane and endoplasmic reticulum, the specific mechanisms of their formation and cellular topology remain largely unknown.10,11 Mounting evidence suggests that different cell types in the plaque deal differently with the cholesterol burden. Akishima et al have recently reported that in human atherosclerotic lesions macrophages are more susceptible than SMCs to apoptosis.12 Guyton et al have shown that antioxidants provide SMCs with partial protection from oxLDL-induced toxicity.13 Yu et al found that macrophages are more susceptible than SMCs to toxicity induced by loading cells with cholesterol from chylomicron remnants.14 In that study, superoxide free radical production was the reported mechanism of toxicity, suggesting that cellular lipid peroxidation is a consequence of excess free cholesterol. Factors that could play a role in the mechanism of the relative resistance of SMCs to cholesterol toxicity include: (1) production of endogenous oxysterols and activation of LXR; (2) degree of lipid peroxidation; and (3) cell-specificity of the signaling pathways downstream of free cholesterol accumulation.
Resistance to the cytotoxic effects of free cholesterol could make SMCs an important regulator of the evolution of the plaque, providing a second wave of protection when cholesterol burden continues to increase and macrophage foam-cells succumb to cholesterol toxicity in the plaque core. When the results of this study are considered in combination with a previous study from this group,15 a compelling argument can be made for the potential of SMCs to step in as functioning phagocytes under conditions where macrophages lose viability. More support for the idea that SMC proliferation provides a second wave of vascular protection from the insult comes from a study of Wolfbauer et al, who produced foam cells by loading SMCs with lipid droplets from lysed macrophage foam cells.16 The presence of extracellular lipid droplets in atherosclerotic lesions in vivo suggests that this in vitro approach may parallel a physiological scenario by which SMC foam cells are produced by ingestion of cellular debris and lipid droplets of macrophage derivation. These studies support the speculation that SMCs may be particularly well suited to clean up the mess that results from the death of macrophage foam cells. Even though it would seem intuitive that freshly recruited monocytes and macrophages have a mandate to perform this role, thus thickening the atheroma with new layers of foam cells, it is possible that SMCs become the phagocyte of choice because of their ability to remain viable under severe lipid stress.
The resistance of SMCs to cholesterol toxicity may represent better compensatory changes in lipid homeostasis compared with macrophages. Compensatory changes in macrophages include phospholipid synthesis and cytoplasmic whorls of newly synthesized phospholipid in membranes, as an increased content in phosphatidylcholine stabilizes membrane cholesterol.17,18 It would be interesting to know whether SMCs use a similar compensatory mechanisms or if they have unique pathways of resistance to cholesterol-induced cytotoxicity.
In recent studies from our laboratory, we have described the atherogenic effects of ACAT-1 deficiency in macrophages.8,19 The study by Rong et al and other studies have confirmed the potential for ACAT inhibition to promote foam cell macrophage death.6,7,9 Rong et al have re-opened the door to the use of ACAT inhibitors in atherogenesis, as SMC proliferation is prevalent in advanced-stage atherosclerosis and in restenosis.20 In light of the current study, it seems that in circumstances where SMCs represent a large portion of the plaque, ACAT-1 inhibition may serve a therapeutic role as direct antiatherosclerotic agent.
ACAT inhibitors have had mixed results, with protective effects in atherosclerosis in vivo and potentially toxic consequences in macrophages in vitro. Resistance of SMC-derived foam cells to cholesterol toxicity provides a working explanation for these results and puts ACAT-1 inhibition targeted to the SMCs in the crosshairs as a potential therapeutic target in atherosclerosis.
Acknowledgments
M.F.L. and S.F. were supported by National Institutes of Health (NIH) grants HL53989, HL65709, HL57986, and HL65405. D.E.D. was supported by NIH training grant HL-07751–08 and an American Heart Association predoctoral fellowship grant.
References
Still WJ. An electron microscope study of cholesterol atherosclerosis in the rabbit. Exp Mol Pathol. 1963; 14: 491–502.
Buck RC. Lesions in the rabbit aorta produced by feeding a high cholesterol diet followed by a normal diet. An electron microscopic study. Br J Exp Pathol. 1962; 43: 236–240.
Buhman KF, Accad M, Farese RV. Mammalian acyl-CoA: cholesterol acyltransferases. Biochim Biophys Acta. 2000; 1529: 142–154.
Alegret M, Llaverias G, Silvestre JS. Acyl coenzyme A: cholesterol acyltransferase inhibitors as hypolipidemic and antiatherosclerotic drugs. Methods Find Exp Clin Pharmacol. 2004; 26: 563–586.
Kusunoki J, Hansoty DK, Aragane K, Fallon JT, Badimon JJ, Fisher EA. Acyl-CoA: cholesterol acyltransferase inhibition reduces atherosclerosis in apolipoprotein E-deficient mice. Circulation. 2001; 103: 2604–2609.
Warner GJ, Stoudt G, Bamberger M, Johnson WJ, Rothblat GH. Cell toxicity induced by inhibition of acyl coenzyme A: cholesterol acyltransferase and accumulation of unesterified cholesterol. J Biol Chem. 1995; 270: 5772–5778.
Kellner-Weibel G, Jerome WG, Small DM, Warner GJ, Stoltenborg JK, Kearney MA, Corjay MH, Phillips MC, Rothblat GH. Effects of intracellular free cholesterol accumulation on macrophage viability: a model for foam cell death. Arterioscler Thromb Vasc Biol. 1998; 18: 423–431.
Fazio S, Major AS, Swift LL, Gleaves LA, Accad M, Linton MF, Farese RV Jr. Increased atherosclerosis in LDL receptor-null mice lacking ACAT1 in macrophages. J Clin Invest. 2001; 107: 163–171.
Rong JX, Kusunoki J, Oelkers P, Sturley SL, Fisher EA. Acyl-CoenzymeA (CoA): Cholesterol Acyltransferase Inhibition in Rat and Human Aortic Smooth Muscle Cells Is Nontoxic and Retards Foam Cell Formation. Arterioscler Thromb Vasc Biol. 2005; 25: 122–127.
Feng B, Yao PM, Li Y, Devlin CM, Zhang D, Harding HP, Sweeney M, Rong JX, Kuriakose G, Fisher EA, Marks AR, Ron D, Tabas I. The endoplasmic reticulum is the site of cholesterol-induced cytotoxicity in macrophages. Nat Cell Biol. 2003; 5: 781–792.
Kellner-Weibel G, Geng YJ, Rothblat GH. Cytotoxic cholesterol is generated by the hydrolysis of cytoplasmic cholesteryl ester and transported to the plasma membrane. Atherosclerosis. 1999; 146: 309–319.
Akishima Y, Akasaka Y, Ishikawa Y, Lijun Z, Kiguchi H, Ito K, Itabe H, Ishii T. Role of macrophage and smooth muscle cell apoptosis in association with oxidized low-density lipoprotein in the atherosclerotic development. Mod Pathol. 2004;Epub 2004 Aug 20.
Guyton JR, Lenz ML, Mathews B, Hughes H, Karsan D, Selinger E, Smith CV. Toxicity of oxidized low density lipoproteins for vascular smooth muscle cells and partial protection by antioxidants. Atherosclerosis. 1995; 118: 237–249.
Yu KC, Mamo JC. Chylomicron-remnant-induced foam cell formation and cytotoxicity: a possible mechanism of cell death in atherosclerosis. Clin Sci (Lond). 2000; 98: 183–192.
Rong JX, Shapiro M, Trogan E, Fisher EA. Transdifferentiation of mouse aortic smooth muscle cells to a macrophage-like state after cholesterol loading. Proc Natl Acad Sci U S A. 2003; 100: 13531–135316.
Wolfbauer G, Glick JM, Minor LK, Rothblat GH. Development of the smooth muscle foam cell: uptake of macrophage lipid inclusions. Proc Natl Acad Sci U S A. 1986; 83: 7760–7764.
Tabas I. Consequences of cellular cholesterol accumulation: basic concepts and physiological implications. J Clin Invest. 2002; 110: 905–911.
Shiratori Y, Okwu AK, Tabas I. Free cholesterol loading of macrophages stimulates phosphatidylcholine biosynthesis and up-regulation of CTP: phosphocholine cytidylyltransferase. J Biol Chem. 1994; 269: 11337–11348.
Dove DE, Su YR, Zhang W, Jerome WG, Swift LL, Linton MF, Fazio S. ACAT1 deficiency disrupts cholesterol efflux and alters cellular morphology in macrophages. Arterioscler Thromb Vasc Biol. 2005; 25: 128–134.
Casscells W. Migration of smooth muscle and endothelial cells. Critical events in restenosis. Circulation. 1992; 86: 723–729.(Sergio Fazio; Dwayne E. D)
Correspondence to Sergio Fazio, Vanderbilt University Medical Center, Division of Cardiovascular Medicine, 383 Preston Research Building, Nashville, TN 37232-6300. E-mail sergio.fazio@vanderbilt.edu
Ahallmark of the atherosclerotic plaque is the accumulation of lipid-laden foam cells that are derived from macrophages or smooth muscle cells (SMCs).1,2 Cholesterol from modified lipoproteins is taken up and either stored in intracellular lipid droplets or delivered to extracellular cholesterol acceptors. Cellular free cholesterol can be exported to nascent HDL and activate the reverse cholesterol transport system, but when it accumulates in large amounts it can induce toxic changes leading to cell death. Conversely, esterified cholesterol is a more inert form of cholesterol for storage, but is not available for efflux to extracellular cholesterol acceptors. Acyl-coenzyme A (CoA): cholesterol acyltransferase (ACAT) esterifies excess free cholesterol and regulates cellular cholesterol homeostasis. ACAT occurs in two isoforms, ACAT-1 and ACAT-2. Whereas liver and intestine express both ACAT-1 and ACAT-2, macrophages and SMCs express ACAT-1 only.3 Nonselective inhibition of ACAT has the potential to both lower plasma lipids and to reduce foam cell formation.3,4 However, the therapeutic potential of ACAT inhibitors is not without controversy, because data have been published suggesting beneficial effects on experimental atherosclerosis as well as negative effects on macrophage viability.5–7 Furthermore, elimination of macrophage ACAT-1 is associated with increased atherosclerotic lesion formation in murine models.8 In this issue of Arteriosclerosis, Thrombosis, and Vascular Biology, Rong et al point to SMCs as a possible explanation for the apparently contradictory effects of ACAT inhibition.9
See page 122
In this study, Rong et al confirm the role of ACAT-1 in the formation of SMC foam cells and show that SMCs are resistant to the toxic effects of ACAT inhibition and free cholesterol (Figure).9 SMCs that were treated with cholesterol cyclodextrin complexes produced foam cells with lipid droplets and elevated levels of cholesteryl esters. The ACAT inhibitor F-1394 was able to block the accumulation of lipid droplets. More importantly, the authors found a significant difference between SMC and macrophages with regard to cholesterol-induced toxicity. ACAT inhibition that caused cholesterol toxicity in cholesterol-loaded macrophages was apparently well tolerated cholesterol-loaded SMC. This finding provides a potential explanation for how ACAT inhibition can cause a decrease in the size of the macrophage foam-cell lesion and an apparent increase in the number of SMCs in the plaque.5
SMC Foam cells are resistant to cholesterol toxicity. Both macrophages and SMCs can assume a foam cell phenotype. Inhibition of ACAT, the enzyme that esterifies intracellular cholesterol, can have toxic consequences for macrophages in vitro. The resistance of SMC foam cells to free cholesterol toxicity may represent the basis for the mixed effects of ACAT inhibition on plaque burden.
These interesting results might have been partly influenced by the unique loading method, which redistributes large amounts of cholesterol to the plasma membrane. It is not known whether cholesterol that enters through the plasma membrane has the same toxic potential as cholesterol that enters by receptor-mediated uptake of modified lipoproteins or phagocytic uptake of cellular debris. As for the resistance of SMC foam cells to toxicity, it is possible that relative to macrophages SMC have less cholesterol routed to toxic pools or more efficient removal of cholesterol from these pools.
The exact nature of cytotoxic pools of cholesterol is unknown. Although these pools are associated with both plasma membrane and endoplasmic reticulum, the specific mechanisms of their formation and cellular topology remain largely unknown.10,11 Mounting evidence suggests that different cell types in the plaque deal differently with the cholesterol burden. Akishima et al have recently reported that in human atherosclerotic lesions macrophages are more susceptible than SMCs to apoptosis.12 Guyton et al have shown that antioxidants provide SMCs with partial protection from oxLDL-induced toxicity.13 Yu et al found that macrophages are more susceptible than SMCs to toxicity induced by loading cells with cholesterol from chylomicron remnants.14 In that study, superoxide free radical production was the reported mechanism of toxicity, suggesting that cellular lipid peroxidation is a consequence of excess free cholesterol. Factors that could play a role in the mechanism of the relative resistance of SMCs to cholesterol toxicity include: (1) production of endogenous oxysterols and activation of LXR; (2) degree of lipid peroxidation; and (3) cell-specificity of the signaling pathways downstream of free cholesterol accumulation.
Resistance to the cytotoxic effects of free cholesterol could make SMCs an important regulator of the evolution of the plaque, providing a second wave of protection when cholesterol burden continues to increase and macrophage foam-cells succumb to cholesterol toxicity in the plaque core. When the results of this study are considered in combination with a previous study from this group,15 a compelling argument can be made for the potential of SMCs to step in as functioning phagocytes under conditions where macrophages lose viability. More support for the idea that SMC proliferation provides a second wave of vascular protection from the insult comes from a study of Wolfbauer et al, who produced foam cells by loading SMCs with lipid droplets from lysed macrophage foam cells.16 The presence of extracellular lipid droplets in atherosclerotic lesions in vivo suggests that this in vitro approach may parallel a physiological scenario by which SMC foam cells are produced by ingestion of cellular debris and lipid droplets of macrophage derivation. These studies support the speculation that SMCs may be particularly well suited to clean up the mess that results from the death of macrophage foam cells. Even though it would seem intuitive that freshly recruited monocytes and macrophages have a mandate to perform this role, thus thickening the atheroma with new layers of foam cells, it is possible that SMCs become the phagocyte of choice because of their ability to remain viable under severe lipid stress.
The resistance of SMCs to cholesterol toxicity may represent better compensatory changes in lipid homeostasis compared with macrophages. Compensatory changes in macrophages include phospholipid synthesis and cytoplasmic whorls of newly synthesized phospholipid in membranes, as an increased content in phosphatidylcholine stabilizes membrane cholesterol.17,18 It would be interesting to know whether SMCs use a similar compensatory mechanisms or if they have unique pathways of resistance to cholesterol-induced cytotoxicity.
In recent studies from our laboratory, we have described the atherogenic effects of ACAT-1 deficiency in macrophages.8,19 The study by Rong et al and other studies have confirmed the potential for ACAT inhibition to promote foam cell macrophage death.6,7,9 Rong et al have re-opened the door to the use of ACAT inhibitors in atherogenesis, as SMC proliferation is prevalent in advanced-stage atherosclerosis and in restenosis.20 In light of the current study, it seems that in circumstances where SMCs represent a large portion of the plaque, ACAT-1 inhibition may serve a therapeutic role as direct antiatherosclerotic agent.
ACAT inhibitors have had mixed results, with protective effects in atherosclerosis in vivo and potentially toxic consequences in macrophages in vitro. Resistance of SMC-derived foam cells to cholesterol toxicity provides a working explanation for these results and puts ACAT-1 inhibition targeted to the SMCs in the crosshairs as a potential therapeutic target in atherosclerosis.
Acknowledgments
M.F.L. and S.F. were supported by National Institutes of Health (NIH) grants HL53989, HL65709, HL57986, and HL65405. D.E.D. was supported by NIH training grant HL-07751–08 and an American Heart Association predoctoral fellowship grant.
References
Still WJ. An electron microscope study of cholesterol atherosclerosis in the rabbit. Exp Mol Pathol. 1963; 14: 491–502.
Buck RC. Lesions in the rabbit aorta produced by feeding a high cholesterol diet followed by a normal diet. An electron microscopic study. Br J Exp Pathol. 1962; 43: 236–240.
Buhman KF, Accad M, Farese RV. Mammalian acyl-CoA: cholesterol acyltransferases. Biochim Biophys Acta. 2000; 1529: 142–154.
Alegret M, Llaverias G, Silvestre JS. Acyl coenzyme A: cholesterol acyltransferase inhibitors as hypolipidemic and antiatherosclerotic drugs. Methods Find Exp Clin Pharmacol. 2004; 26: 563–586.
Kusunoki J, Hansoty DK, Aragane K, Fallon JT, Badimon JJ, Fisher EA. Acyl-CoA: cholesterol acyltransferase inhibition reduces atherosclerosis in apolipoprotein E-deficient mice. Circulation. 2001; 103: 2604–2609.
Warner GJ, Stoudt G, Bamberger M, Johnson WJ, Rothblat GH. Cell toxicity induced by inhibition of acyl coenzyme A: cholesterol acyltransferase and accumulation of unesterified cholesterol. J Biol Chem. 1995; 270: 5772–5778.
Kellner-Weibel G, Jerome WG, Small DM, Warner GJ, Stoltenborg JK, Kearney MA, Corjay MH, Phillips MC, Rothblat GH. Effects of intracellular free cholesterol accumulation on macrophage viability: a model for foam cell death. Arterioscler Thromb Vasc Biol. 1998; 18: 423–431.
Fazio S, Major AS, Swift LL, Gleaves LA, Accad M, Linton MF, Farese RV Jr. Increased atherosclerosis in LDL receptor-null mice lacking ACAT1 in macrophages. J Clin Invest. 2001; 107: 163–171.
Rong JX, Kusunoki J, Oelkers P, Sturley SL, Fisher EA. Acyl-CoenzymeA (CoA): Cholesterol Acyltransferase Inhibition in Rat and Human Aortic Smooth Muscle Cells Is Nontoxic and Retards Foam Cell Formation. Arterioscler Thromb Vasc Biol. 2005; 25: 122–127.
Feng B, Yao PM, Li Y, Devlin CM, Zhang D, Harding HP, Sweeney M, Rong JX, Kuriakose G, Fisher EA, Marks AR, Ron D, Tabas I. The endoplasmic reticulum is the site of cholesterol-induced cytotoxicity in macrophages. Nat Cell Biol. 2003; 5: 781–792.
Kellner-Weibel G, Geng YJ, Rothblat GH. Cytotoxic cholesterol is generated by the hydrolysis of cytoplasmic cholesteryl ester and transported to the plasma membrane. Atherosclerosis. 1999; 146: 309–319.
Akishima Y, Akasaka Y, Ishikawa Y, Lijun Z, Kiguchi H, Ito K, Itabe H, Ishii T. Role of macrophage and smooth muscle cell apoptosis in association with oxidized low-density lipoprotein in the atherosclerotic development. Mod Pathol. 2004;Epub 2004 Aug 20.
Guyton JR, Lenz ML, Mathews B, Hughes H, Karsan D, Selinger E, Smith CV. Toxicity of oxidized low density lipoproteins for vascular smooth muscle cells and partial protection by antioxidants. Atherosclerosis. 1995; 118: 237–249.
Yu KC, Mamo JC. Chylomicron-remnant-induced foam cell formation and cytotoxicity: a possible mechanism of cell death in atherosclerosis. Clin Sci (Lond). 2000; 98: 183–192.
Rong JX, Shapiro M, Trogan E, Fisher EA. Transdifferentiation of mouse aortic smooth muscle cells to a macrophage-like state after cholesterol loading. Proc Natl Acad Sci U S A. 2003; 100: 13531–135316.
Wolfbauer G, Glick JM, Minor LK, Rothblat GH. Development of the smooth muscle foam cell: uptake of macrophage lipid inclusions. Proc Natl Acad Sci U S A. 1986; 83: 7760–7764.
Tabas I. Consequences of cellular cholesterol accumulation: basic concepts and physiological implications. J Clin Invest. 2002; 110: 905–911.
Shiratori Y, Okwu AK, Tabas I. Free cholesterol loading of macrophages stimulates phosphatidylcholine biosynthesis and up-regulation of CTP: phosphocholine cytidylyltransferase. J Biol Chem. 1994; 269: 11337–11348.
Dove DE, Su YR, Zhang W, Jerome WG, Swift LL, Linton MF, Fazio S. ACAT1 deficiency disrupts cholesterol efflux and alters cellular morphology in macrophages. Arterioscler Thromb Vasc Biol. 2005; 25: 128–134.
Casscells W. Migration of smooth muscle and endothelial cells. Critical events in restenosis. Circulation. 1992; 86: 723–729.(Sergio Fazio; Dwayne E. D)