Background Lipoprotein receptors from the low density lipoprotein (LDL) receptor family are multifunctional membrane proteins which can efficiently mediate endocytosis and thereby facilitate lipoprotein clearance from the plasma. The biggest member of this family, the LDL receptor-related protein 1 (LRP1), facilitates the hepatic uptake of triglyceride-rich lipoproteins (TRL) via interaction with apolipoprotein E (apoE). In contrast to the classical LDL degradation pathway, TRL disintegrate in peripheral endosomes, and core lipids and apoB are targeted along the endocytic pathway for lysosomal degradation. Notably, TRL-derived apoE remains within recycling endosomes and is then mobilized by high density lipoproteins (HDL) for re-secretion. The aim of this study is to investigate the involvement of LRP1 in the regulation of apoE recycling. Principal Findings Immunofluorescence studies indicate the LRP1-dependent trapping of apoE in EEA1-positive endosomes in human hepatoma cells. This processing is distinct from other LRP1 ligands such as RAP which is efficiently targeted to lysosomal compartments. Upon stimulation of HDL-induced recycling, apoE is released from LRP1-positive endosomes but is targeted to another, distinct population of early endosomes that contain HDL, but not LRP1. For subsequent analysis of the recycling capacity, we expressed the full-length human LRP1 and used an RNA interference approach to manipulate the expression levels of LRP1. In support of LRP1 determining the intracellular fate of apoE, overexpression of LRP1 significantly stimulated HDL-induced apoE recycling. Vice versa LRP1 knockdown in HEK293 cells and primary hepatocytes strongly reduced the efficiency of HDL to stimulate apoE secretion. Conclusion We conclude that LRP1 enables apoE to accumulate in an early endosomal recycling compartment that serves as a pool for the intracellular formation and subsequent re-secretion of apoE-enriched HDL particles.
References
[1]
Merkel M, Eckel RH, Goldberg IJ (2002) Lipoprotein lipase: genetics, lipid uptake, and regulation. J Lipid Res 43: 1997–2006.
[2]
Bartelt A, Bruns OT, Reimer R, Hohenberg H, Ittrich H, et al. (2011) Brown adipose tissue activity controls triglyceride clearance. Nat Med 17: 200–205. nm.2297 [pii];10.1038/nm.2297.
[3]
Heeren J, Beisiegel U, Grewal T (2006) Apolipoprotein E recycling: implications for dyslipidemia and atherosclerosis. Arterioscler Thromb Vasc Biol 26: 442–448.
[4]
Beisiegel U, Weber W, Ihrke G, Herz J, Stanley KK (1989) The LDL-receptor-related protein, LRP, is an apolipoprotein E-binding protein. Nature 341: 162–164.
[5]
Beisiegel U, Weber W, Bengtsson-Olivecrona G (1991) Lipoprotein lipase enhances the binding of chylomicrons to low density lipoprotein receptor-related protein. Proc Natl Acad Sci U S A 88: 8342–8346.
[6]
Rohlmann A, Gotthardt M, Hammer RE, Herz J (1998) Inducible inactivation of hepatic LRP gene by cre-mediated recombination confirms role of LRP in clearance of chylomicron remnants. J Clin Invest 101: 689–695.
[7]
Laatsch A, Merkel M, Talmud PJ, Grewal T, Beisiegel U, et al. (2008) Insulin stimulates hepatic low density lipoprotein receptor-related protein 1 (LRP1) to increase postprandial lipoprotein clearance. Atherosclerosis.
[8]
Stanford KI, Bishop JR, Foley EM, Gonzales JC, Niesman IR, et al. (2009) Syndecan-1 is the primary heparan sulfate proteoglycan mediating hepatic clearance of triglyceride-rich lipoproteins in mice. J Clin Invest 119: 3236–3245. 38251 [pii];10.1172/JCI38251.
[9]
Williams KJ, Chen K (2010) Recent insights into factors affecting remnant lipoprotein uptake. Curr Opin Lipidol 21: 218–228. 10.1097/MOL.0b013e328338cabc [doi];00041433-201006000-00010 [pii].
[10]
Havel RJ, Hamilton RL (2004) Hepatic catabolism of remnant lipoproteins: where the action is. Arterioscler Thromb Vasc Biol 24: 213–215.
[11]
MacArthur JM, Bishop JR, Stanford KI, Wang L, Bensadoun A, et al. (2007) Liver heparan sulfate proteoglycans mediate clearance of triglyceride-rich lipoproteins independently of LDL receptor family members. J Clin Invest 117: 153–164.
[12]
Out R, Kruijt JK, Rensen PC, Hildebrand RB, de Vos P, et al. (2004) Scavenger receptor BI plays a role in facilitating chylomicron metabolism. J Biol Chem 279: 18401–18406.
[13]
Heeren J, Weber W, Beisiegel U (1999) Intracellular processing of endocytosed triglyceride-rich lipoproteins comprises both recycling and degradation. J Cell Sci 112(Pt 3): 349–359.
[14]
Rensen PC, Jong MC, van Vark LC, van der BH, Hendriks WL, et al. (2000) Apolipoprotein E is resistant to intracellular degradation in vitro and in vivo. Evidence for retroendocytosis. J Biol Chem 275: 8564–8571.
[15]
Heeren J, Grewal T, Jackle S, Beisiegel U (2001) Recycling of apolipoprotein E and lipoprotein lipase through endosomal compartments in vivo. J Biol Chem 276: 42333–42338.
[16]
Swift LL, Farkas MH, Major AS, Valyi-Nagy K, Linton MF, et al. (2001) A recycling pathway for resecretion of internalized apolipoprotein E in liver cells. J Biol Chem 276: 22965–22970.
[17]
Heeren J, Grewal T, Laatsch A, Rottke D, Rinninger F, et al. (2003) Recycling of apoprotein E is associated with cholesterol efflux and high density lipoprotein internalization. J Biol Chem 278: 14370–14378.
[18]
Hasty AH, Plummer MR, Weisgraber KH, Linton MF, Fazio S, et al. (2005) The recycling of apolipoprotein E in macrophages: Influence of HDL and apolipoprotein AI. J Lipid Res 46: 1433–1439.
[19]
Heeren J, Grewal T, Laatsch A, Becker N, Rinninger F, et al. (2004) Impaired recycling of apolipoprotein E4 is associated with intracellular cholesterol accumulation. J Biol Chem 279: 55483–55492.
[20]
Rellin L, Heeren J, Beisiegel U (2008) Recycling of apolipoprotein E is not associated with cholesterol efflux in neuronal cells. Biochim Biophys Acta 1781: 232–238.
[21]
Davignon J, Gregg RE, Sing CF (1988) Apolipoprotein E polymorphism and atherosclerosis. Arteriosclerosis 8: 1–21.
[22]
Roses AD (1996) Apolipoprotein E alleles as risk factors in Alzheimer's disease. Annu Rev Med 47: 387–400.
[23]
Heeren J, Niemeier A, Merkel M, Beisiegel U (2002) Endothelial-derived lipoprotein lipase is bound to postprandial triglyceride-rich lipoproteins and mediates their hepatic clearance in vivo. J Mol Med 80: 576–584.
[24]
Binder RJ, Han DK, Srivastava PK (2000) CD91: a receptor for heat shock protein gp96. Nat Immunol 1: 151–155.
[25]
Binder RJ, Srivastava PK (2004) Essential role of CD91 in re-presentation of gp96-chaperoned peptides. Proc Natl Acad Sci U S A 101: 6128–6133.
[26]
Herz J, Goldstein JL, Strickland DK, Ho YK, Brown MS (1991) 39-kDa protein modulates binding of ligands to low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor. J Biol Chem 266: 21232–21238.
[27]
Willnow TE, Orth K, Herz J (1994) Molecular dissection of ligand binding sites on the low density lipoprotein receptor-related protein. J Biol Chem 269: 15827–15832.
[28]
Herz J, Strickland DK (2001) LRP: a multifunctional scavenger and signaling receptor. J Clin Invest 108: 779–784.
[29]
Lillis AP, Mikhailenko I, Strickland DK (2005) Beyond endocytosis: LRP function in cell migration, proliferation and vascular permeability. J Thromb Haemost 3: 1884–1893.
[30]
Harasaki K, Lubben NB, Harbour M, Taylor MJ, Robinson MS (2005) Sorting of major cargo glycoproteins into clathrin-coated vesicles. Traffic 6: 1014–1026.
[31]
Laatsch A, Ragozin S, Grewal T, Beisiegel U, Heeren J (2004) Differential RNA interference: replacement of endogenous with recombinant low density lipoprotein receptor-related protein (LRP). Eur J Cell Biol 83: 113–120.
[32]
Willnow TE, Moehring JM, Inocencio NM, Moehring TJ, Herz J (1996) The low-density-lipoprotein receptor-related protein (LRP) is processed by furin in vivo and in vitro. Biochem J 313(Pt 1): 71–76.
[33]
Czekay RP, Orlando RA, Woodward L, Lundstrom M, Farquhar MG (1997) Endocytic trafficking of megalin/RAP complexes: dissociation of the complexes in late endosomes. Mol Biol Cell 8: 517–532.
[34]
Willnow TE, Goldstein JL, Orth K, Brown MS, Herz J (1992) Low density lipoprotein receptor-related protein and gp330 bind similar ligands, including plasminogen activator-inhibitor complexes and lactoferrin, an inhibitor of chylomicron remnant clearance. J Biol Chem 267: 26172–26180.
[35]
Muller D, Nykjaer A, Willnow TE (2003) From holoprosencephaly to osteopathology: role of multifunctional endocytic receptors in absorptive epithelia. Ann Med 35: 290–299.
[36]
Terrand J, Bruban V, Zhou L, Gong W, El Asmar Z, et al. (2009) LRP1 controls intracellular cholesterol storage and fatty acid synthesis through modulation of Wnt signaling. J Biol Chem 284: 381–8.
[37]
Boucher P, Li WP, Matz RL, Takayama Y, Auwerx J, et al. (2007) LRP1 functions as an atheroprotective integrator of TGFbeta and PDFG signals in the vascular wall: implications for Marfan syndrome. PLoS ONE 2: e448.
[38]
Boucher P, Gotthardt M, Li WP, Anderson RG, Herz J (2003) LRP: role in vascular wall integrity and protection from atherosclerosis. Science 300: 329–332.
[39]
Chen Y, Durakoglugil MS, Xian X, Herz J (2010) ApoE4 reduces glutamate receptor function and synaptic plasticity by selectively impairing ApoE receptor recycling. Proc Natl Acad Sci U S A 107: 12011–12016.
[40]
Meredith MJ (1988) Rat hepatocytes prepared without collagenase: prolonged retention of differentiated characteristics in culture. Cell Biol Toxicol 4: 405–425.
[41]
Kounnas MZ, Argraves WS, Strickland DK (1992) The 39-kDa receptor-associated protein interacts with two members of the low density lipoprotein receptor family, alpha 2-macroglobulin receptor and glycoprotein 330. J Biol Chem 267: 21162–21166.
