Unique Pathway for Cholesterol Uptake in Fat Cells
http://www.100md.com
《动脉硬化血栓血管生物学》
From the Division of Cardiovascular Medicine, Department of Medicine (S.F., M.F.L.), and the Departments of Pathology (S.F.) and Pharmacology (M.F.L.), Vanderbilt University Medical Center, Nashville, Tenn.
The biologic features of high-density lipoprotein (HDL) cholesterol delivery to the cell have mostly been investigated in hepatocytes, the consensus termination point of the reverse cholesterol transport pathway. It has long been known that the delivery of HDL cholesterol to the hepatocyte does not involve the internalization and degradation of the lipoprotein particle, as is the case for the apolipoprotein (apo)B-containing lipoproteins, but rather it is based on the selective unloading of the cholesterol cargo from the particle into the cell.1 With the discovery that the scavenger receptor type BI (SR-BI) acts as the hepatic HDL receptor,2 the final events of HDL cholesterol delivery have been well characterized: HDL binds to SR-BI through apolipoprotein AI (apoAI), the cholesteryl ester (CE) cargo of HDL is selectively unloaded into the cell where it is trafficked back into forming lipoproteins or into the bile, and the lipid-poor apoAI returns in the circulation for another round of peripheral cholesterol collection.3 Even though this appears to be a systemic mechanism for redistribution of peripheral cholesterol to a central organ in charge of cholesterol disposal, it is possible that alternative processes might have developed so that particular non-hepatic cell types can acquire needed cholesterol from the HDL.
See page 1669
Adipocytes might be unique in their need of acquiring cholesterol, because of their limited ability to synthesize their own.4 This can be caused by a preferential utilization of acetate for fatty acid synthesis or reduced efficiency of enzymes downstream of squalene synthase in the pathway of cholesterol synthesis.5 In addition, the membrane pool of fat cells might need large amounts of cholesterol at times of rapidly expanding adipocyte size, both to maintain cellular integrity and to regulate cellular hypertrophy.6 For this reason, the adipocyte may have developed multiple and partially redundant ways to extract cholesterol from circulating lipoproteins such as the HDL. Because SR-BI is abundantly expressed by adipocytes, it is likely that significant amounts of cholesterol are taken in by these cells through the classic HDL delivery mechanism. In this issue of Arteriosclerosis, Thrombosis, and Vascular Biology, an article from Drs Vassiliou and McPherson reports on the detailed mechanism of an alternative unique pathway for selective uptake of HDL-derived CE in fat cells.7 Previous work done in the authors’ laboratory had shown that the uptake of HDL cholesterol in adipocytes occurs in part through noncanonical pathways. For example, they showed that CE transfer protein (CETP) expressed by the adipocyte is involved in the selective extraction of CE from the HDL, and that the transfer of CE can be increased by the use of additional CETP and reduced by the use of an inhibitory antibody against CETP.8 In other studies, they showed that the low-density lipoprotein (LDL) receptor related protein (LRP) is involved in this process as well,9 a surprising notion given that LRP is an internalizing receptor fully capable of driving apoB-containing remnant lipoproteins through the endocytic pathway to a destiny of complete lysosomal degradation. Interestingly, LRP needs the presence of a mediator to perform its function in HDL cholesterol extraction from the cell. This mediator, which is present in adipocytes but absent in skin fibroblasts, appears to be apoE, a well-recognized apoprotein that influences the processing and clearance of plasma lipoprotein cholesterol and also influences intracellular cholesterol homeostasis in several cell types.10
In the current study, the authors have painstakingly defined the mechanisms for this SR-BI–independent CE selective uptake from HDL in adipocytes. Using both primary adipocytes as well as immortalized liposarcoma cells (SW872), the authors show that this process, responsible for one third of the selective uptake of CE from HDL in fat cells, is fully dependent on the cooperation between apoE and LRP. Indeed, this process: (1) does not normally occur in skin fibroblasts but can be induced by incubating cells with extracellular apoE; (2) is abolished in adipocytes from apoE-null mice; and (3) is abrogated by the effect of the receptor associated protein (RAP) in skin fibroblasts only in the presence of exogenous apoE. It is intriguing that several other interventions affecting LRP-mediated internalization reduced the selective uptake of CE HDL from adipocytes to the level seen in apoE-deficient conditions. The fact that extracellular apoE is able to activate the LRP-dependent pathway indicates that apoE probably works through its positioning on the cell membrane. The authors also show that the CE that transfers from the HDL to the adipocyte through this pathway follow a dual destiny; first they are distributed throughout the cell membrane in a reversible compartment that can be re-extracted by the presence of extracellular HDL, and then eventually they are positioned in an intracellular compartment not amenable to exchange with HDL in the media. Most of the CE in the reversible compartment are redirected into the extracellular space by a process involving apoE. It is the newly formed particle of apoE and HDL-derived CE that is finally captured by the LRP for a more committed internalization and for distribution to deeper intracellular compartments.
Putting all the data together, including results from the authors’ previous studies, one could create a scenario where the mature HDL on the surface of the adipocyte starts delivering CE to the membrane through the CETP either present on the lipoprotein surface or secreted by the adipocyte. The CE is distributed throughout the membrane without hydrolysis and is then collected by the apoE on the cell surface for recapture and internalization through LRP to eventually produce free cholesterol destined to a more intricate cellular trafficking (Figure). Considering the important role of apoE in cholesterol sensing,11 it is tempting to speculate that this novel "efflux recapture" process controls the most delicate quota of membrane cholesterol involved in cell size signaling.12 It is not known whether these particles captured by LRP will allow apoE recycling, a phenomenon that has been shown in both hepatocytes and macrophages and that occurs for apoE on the surface of either HDL or VLDL.13–15
In the adipocyte, delivery of HDL cholesterol occurs through both SR-BI–dependent and SR-BI–independent mechanisms. The former, likely responsible for the majority of cholesterol acquisition by the cell, is based on the docking of the HDL to the SR-BI and transfer of CE to the cytoplasma. The SR-BI–independent mechanism starts with CE transfer, probably mediated by CETP, from the HDL to the plasma membrane, and continues with the apoE-mediated resecretion (efflux) of the HDL CE followed by its final uptake (recapture) by LRP. This mechanism may be responsible for one third of cholesterol acquisition by the adipocyte.
As is the case with any strong piece of science, this article raises several questions. These include: (1) What is the role and quantitative contribution of HDL CE uptake from adipocytes in the overall reverse cholesterol transport pathway? (2) Is there a functional difference between HDL cholesterol delivered to the adipocyte through either SR-BI or the "efflux-recapture" process? (3) Will the inhibition of CETP, likely to become a therapeutic strategy to raise HDL in the near future, influence the distribution of cholesterol to the adipocytes and ultimately affect cellular plasticity and function? (4) Does the cholesterol content of the adipocyte play a role in the development of the insulin resistance syndrome?
In summary, it seems that fat cells consider cholesterol more as treasure than as trash, because they have developed a dual mechanism to extract CE from HDL; one is based on the typical HDL receptor SR-BI, the other is based on a novel "efflux recapture" process that needs apoE and the LRP. Because apoE and LRP are linked in secretion recapture loops also in hepatocytes and macrophages, there may be a biological advantage for the LRP-mediated entry of cholesterol into the cell.
Acknowledgments
This work was supported in part by National Institutes of Health grants HL57986, HL65709, HL65405, HL53989, and HL68114.
References
Glass C, Pittman RC, Weinstein DB, Steinberg D. Dissociation of tissue uptake of cholesterol ester from that of apoprotein A-I of rat plasma high density lipoprotein: selective delivery of cholesterol ester to liver, adrenal, and gonad. Proc Natl Acad Sci U S A. 1983; 80: 5435–5439.
Acton S, Rigotti A, Landschulz KT, Xu S, Hobbs HH, Krieger M. Identification of scavenger receptor SR-BI as a high density lipoprotein receptor. Science. 1996; 271: 518–520.
Trigatti B, Rigotti A, Krieger M. The role of the high-density lipoprotein receptor SR-BI in cholesterol metabolism. Curr Opin Lipidol. 2000; 11: 123–131.
Kovanen PT, Nikkila EA, Miettinen TA. Regulation of cholesterol synthesis and storage in fat cells. J Lipid Res. 1975; 16: 211–223.
Tilvis RS, Kovanen PT, Miettinen TA. Release of newly synthesized squalene, methyl sterols and cholesterol from human adipocytes in the presence of lipoproteins. Scand J Clin Lab Invest. 1978; 38: 83–87.
Krause BR, Hartman AD. Adipose tissue and cholesterol metabolism. J Lipid Res. 1984; 25: 97–110.
Vassiliou G, McPherson R. A novel efflux-recapture process underlies the mechanism of high-density lipoprotein cholesteryl ester-selective uptake mediated by the low-density lipoprotein receptor-related protein. Arterioscler Thromb Vasc Biol. 2004; 24: 1669–1675.
Vassiliou G, McPherson R. Role of cholesteryl ester transfer protein in selective uptake of high density lipoprotein cholesteryl esters by adipocytes. J Lipid Res. 2004; In press.
Vassiliou G, Benoist F, Lau P, Kavaslar GN, McPherson R. The low density lipoprotein receptor-related protein contributes to selective uptake of high density lipoprotein cholesteryl esters by SW872 liposarcoma cells and primary human adipocytes. J Biol Chem. 2001; 276: 48823–48830.
Mahley RW, Boyles JK, Pitas RE. Role of apolipoprotein E and high density lipoproteins in lipid redistribution locally within tissues. In: Miller NE, ed. High Density Lipoproteins and Atherosclerosis II. Amsterdam: Elsevier Science Publishers; 1989.
Laffitte BA, Repa JJ, Joseph SB, Wilpitz DC, Kast HR, Mangelsdorf DJ, Tontonoz P. LXRs control lipid-inducible expression of the apolipoprotein E gene in macrophages and adipocytes. Proc Natl Acad Sci U S A. 2001; 98: 507–512.
Dugail I, Le Lay S, Varret M, Le Liepvre X, Dagher G, Ferre P. New insights into how adipocytes sense their triglyceride stores. Is cholesterol a signal? Horm Metab Res. 2003; 35: 204–210.
Fazio S, Linton MF, Hasty AH, Swift LL. Recycling of apolipoprotein E in mouse liver. J Biol Chem. 1999; 274: 8247–8253.
Swift LL, Farkas MH, Major AS, Valyi-Nagy K, Linton MF, Fazio S. A recycling pathway for resecretion of internalized apolipoprotein E in liver cells. J Biol Chem. 2001; 276: 22965–22970.
Farkas MH, Swift LL, Hasty AH, Linton MF, Fazio S. The recycling of apolipoprotein E in primary cultures of mouse hepatocytes. Evidence for a physiologic connection to high density lipoprotein metabolism. J Biol Chem. 2003; 278: 9412–9417.(Sergio Fazio; MacRae F. L)
The biologic features of high-density lipoprotein (HDL) cholesterol delivery to the cell have mostly been investigated in hepatocytes, the consensus termination point of the reverse cholesterol transport pathway. It has long been known that the delivery of HDL cholesterol to the hepatocyte does not involve the internalization and degradation of the lipoprotein particle, as is the case for the apolipoprotein (apo)B-containing lipoproteins, but rather it is based on the selective unloading of the cholesterol cargo from the particle into the cell.1 With the discovery that the scavenger receptor type BI (SR-BI) acts as the hepatic HDL receptor,2 the final events of HDL cholesterol delivery have been well characterized: HDL binds to SR-BI through apolipoprotein AI (apoAI), the cholesteryl ester (CE) cargo of HDL is selectively unloaded into the cell where it is trafficked back into forming lipoproteins or into the bile, and the lipid-poor apoAI returns in the circulation for another round of peripheral cholesterol collection.3 Even though this appears to be a systemic mechanism for redistribution of peripheral cholesterol to a central organ in charge of cholesterol disposal, it is possible that alternative processes might have developed so that particular non-hepatic cell types can acquire needed cholesterol from the HDL.
See page 1669
Adipocytes might be unique in their need of acquiring cholesterol, because of their limited ability to synthesize their own.4 This can be caused by a preferential utilization of acetate for fatty acid synthesis or reduced efficiency of enzymes downstream of squalene synthase in the pathway of cholesterol synthesis.5 In addition, the membrane pool of fat cells might need large amounts of cholesterol at times of rapidly expanding adipocyte size, both to maintain cellular integrity and to regulate cellular hypertrophy.6 For this reason, the adipocyte may have developed multiple and partially redundant ways to extract cholesterol from circulating lipoproteins such as the HDL. Because SR-BI is abundantly expressed by adipocytes, it is likely that significant amounts of cholesterol are taken in by these cells through the classic HDL delivery mechanism. In this issue of Arteriosclerosis, Thrombosis, and Vascular Biology, an article from Drs Vassiliou and McPherson reports on the detailed mechanism of an alternative unique pathway for selective uptake of HDL-derived CE in fat cells.7 Previous work done in the authors’ laboratory had shown that the uptake of HDL cholesterol in adipocytes occurs in part through noncanonical pathways. For example, they showed that CE transfer protein (CETP) expressed by the adipocyte is involved in the selective extraction of CE from the HDL, and that the transfer of CE can be increased by the use of additional CETP and reduced by the use of an inhibitory antibody against CETP.8 In other studies, they showed that the low-density lipoprotein (LDL) receptor related protein (LRP) is involved in this process as well,9 a surprising notion given that LRP is an internalizing receptor fully capable of driving apoB-containing remnant lipoproteins through the endocytic pathway to a destiny of complete lysosomal degradation. Interestingly, LRP needs the presence of a mediator to perform its function in HDL cholesterol extraction from the cell. This mediator, which is present in adipocytes but absent in skin fibroblasts, appears to be apoE, a well-recognized apoprotein that influences the processing and clearance of plasma lipoprotein cholesterol and also influences intracellular cholesterol homeostasis in several cell types.10
In the current study, the authors have painstakingly defined the mechanisms for this SR-BI–independent CE selective uptake from HDL in adipocytes. Using both primary adipocytes as well as immortalized liposarcoma cells (SW872), the authors show that this process, responsible for one third of the selective uptake of CE from HDL in fat cells, is fully dependent on the cooperation between apoE and LRP. Indeed, this process: (1) does not normally occur in skin fibroblasts but can be induced by incubating cells with extracellular apoE; (2) is abolished in adipocytes from apoE-null mice; and (3) is abrogated by the effect of the receptor associated protein (RAP) in skin fibroblasts only in the presence of exogenous apoE. It is intriguing that several other interventions affecting LRP-mediated internalization reduced the selective uptake of CE HDL from adipocytes to the level seen in apoE-deficient conditions. The fact that extracellular apoE is able to activate the LRP-dependent pathway indicates that apoE probably works through its positioning on the cell membrane. The authors also show that the CE that transfers from the HDL to the adipocyte through this pathway follow a dual destiny; first they are distributed throughout the cell membrane in a reversible compartment that can be re-extracted by the presence of extracellular HDL, and then eventually they are positioned in an intracellular compartment not amenable to exchange with HDL in the media. Most of the CE in the reversible compartment are redirected into the extracellular space by a process involving apoE. It is the newly formed particle of apoE and HDL-derived CE that is finally captured by the LRP for a more committed internalization and for distribution to deeper intracellular compartments.
Putting all the data together, including results from the authors’ previous studies, one could create a scenario where the mature HDL on the surface of the adipocyte starts delivering CE to the membrane through the CETP either present on the lipoprotein surface or secreted by the adipocyte. The CE is distributed throughout the membrane without hydrolysis and is then collected by the apoE on the cell surface for recapture and internalization through LRP to eventually produce free cholesterol destined to a more intricate cellular trafficking (Figure). Considering the important role of apoE in cholesterol sensing,11 it is tempting to speculate that this novel "efflux recapture" process controls the most delicate quota of membrane cholesterol involved in cell size signaling.12 It is not known whether these particles captured by LRP will allow apoE recycling, a phenomenon that has been shown in both hepatocytes and macrophages and that occurs for apoE on the surface of either HDL or VLDL.13–15
In the adipocyte, delivery of HDL cholesterol occurs through both SR-BI–dependent and SR-BI–independent mechanisms. The former, likely responsible for the majority of cholesterol acquisition by the cell, is based on the docking of the HDL to the SR-BI and transfer of CE to the cytoplasma. The SR-BI–independent mechanism starts with CE transfer, probably mediated by CETP, from the HDL to the plasma membrane, and continues with the apoE-mediated resecretion (efflux) of the HDL CE followed by its final uptake (recapture) by LRP. This mechanism may be responsible for one third of cholesterol acquisition by the adipocyte.
As is the case with any strong piece of science, this article raises several questions. These include: (1) What is the role and quantitative contribution of HDL CE uptake from adipocytes in the overall reverse cholesterol transport pathway? (2) Is there a functional difference between HDL cholesterol delivered to the adipocyte through either SR-BI or the "efflux-recapture" process? (3) Will the inhibition of CETP, likely to become a therapeutic strategy to raise HDL in the near future, influence the distribution of cholesterol to the adipocytes and ultimately affect cellular plasticity and function? (4) Does the cholesterol content of the adipocyte play a role in the development of the insulin resistance syndrome?
In summary, it seems that fat cells consider cholesterol more as treasure than as trash, because they have developed a dual mechanism to extract CE from HDL; one is based on the typical HDL receptor SR-BI, the other is based on a novel "efflux recapture" process that needs apoE and the LRP. Because apoE and LRP are linked in secretion recapture loops also in hepatocytes and macrophages, there may be a biological advantage for the LRP-mediated entry of cholesterol into the cell.
Acknowledgments
This work was supported in part by National Institutes of Health grants HL57986, HL65709, HL65405, HL53989, and HL68114.
References
Glass C, Pittman RC, Weinstein DB, Steinberg D. Dissociation of tissue uptake of cholesterol ester from that of apoprotein A-I of rat plasma high density lipoprotein: selective delivery of cholesterol ester to liver, adrenal, and gonad. Proc Natl Acad Sci U S A. 1983; 80: 5435–5439.
Acton S, Rigotti A, Landschulz KT, Xu S, Hobbs HH, Krieger M. Identification of scavenger receptor SR-BI as a high density lipoprotein receptor. Science. 1996; 271: 518–520.
Trigatti B, Rigotti A, Krieger M. The role of the high-density lipoprotein receptor SR-BI in cholesterol metabolism. Curr Opin Lipidol. 2000; 11: 123–131.
Kovanen PT, Nikkila EA, Miettinen TA. Regulation of cholesterol synthesis and storage in fat cells. J Lipid Res. 1975; 16: 211–223.
Tilvis RS, Kovanen PT, Miettinen TA. Release of newly synthesized squalene, methyl sterols and cholesterol from human adipocytes in the presence of lipoproteins. Scand J Clin Lab Invest. 1978; 38: 83–87.
Krause BR, Hartman AD. Adipose tissue and cholesterol metabolism. J Lipid Res. 1984; 25: 97–110.
Vassiliou G, McPherson R. A novel efflux-recapture process underlies the mechanism of high-density lipoprotein cholesteryl ester-selective uptake mediated by the low-density lipoprotein receptor-related protein. Arterioscler Thromb Vasc Biol. 2004; 24: 1669–1675.
Vassiliou G, McPherson R. Role of cholesteryl ester transfer protein in selective uptake of high density lipoprotein cholesteryl esters by adipocytes. J Lipid Res. 2004; In press.
Vassiliou G, Benoist F, Lau P, Kavaslar GN, McPherson R. The low density lipoprotein receptor-related protein contributes to selective uptake of high density lipoprotein cholesteryl esters by SW872 liposarcoma cells and primary human adipocytes. J Biol Chem. 2001; 276: 48823–48830.
Mahley RW, Boyles JK, Pitas RE. Role of apolipoprotein E and high density lipoproteins in lipid redistribution locally within tissues. In: Miller NE, ed. High Density Lipoproteins and Atherosclerosis II. Amsterdam: Elsevier Science Publishers; 1989.
Laffitte BA, Repa JJ, Joseph SB, Wilpitz DC, Kast HR, Mangelsdorf DJ, Tontonoz P. LXRs control lipid-inducible expression of the apolipoprotein E gene in macrophages and adipocytes. Proc Natl Acad Sci U S A. 2001; 98: 507–512.
Dugail I, Le Lay S, Varret M, Le Liepvre X, Dagher G, Ferre P. New insights into how adipocytes sense their triglyceride stores. Is cholesterol a signal? Horm Metab Res. 2003; 35: 204–210.
Fazio S, Linton MF, Hasty AH, Swift LL. Recycling of apolipoprotein E in mouse liver. J Biol Chem. 1999; 274: 8247–8253.
Swift LL, Farkas MH, Major AS, Valyi-Nagy K, Linton MF, Fazio S. A recycling pathway for resecretion of internalized apolipoprotein E in liver cells. J Biol Chem. 2001; 276: 22965–22970.
Farkas MH, Swift LL, Hasty AH, Linton MF, Fazio S. The recycling of apolipoprotein E in primary cultures of mouse hepatocytes. Evidence for a physiologic connection to high density lipoprotein metabolism. J Biol Chem. 2003; 278: 9412–9417.(Sergio Fazio; MacRae F. L)