Transcytosis of Lipoprotein Lipase across Cultured Endothelial Cells Requires Both Heparan Sulfate Proteoglycans and the Very Low Density Lipoprotein Receptor

Abstract

Lipoprotein lipase (LPL), the major enzyme responsible for the hydrolysis of circulating lipoprotein triglyceride molecules, is synthesized in myocytes and adipocytes but functions while bound to heparan sulfate proteoglycans (HSPGs) on the luminal surface of vascular endothelial cells. This requires transfer of LPL from the abluminal side to the luminal side of endothelial cells. Studies were performed to investigate the mechanisms of LPL transcytosis using cultured monolayers of bovine aortic endothelial cells. We tested whether HSPGs and members of the low density lipoprotein (LDL) receptor superfamily were involved in transfer of LPL from the basolateral to the apical side of cultured endothelial cells. Heparinase/heparinitase treatment of the basolateral cell surface or addition of heparin to the basolateral medium decreased the movement of LPL. This suggested a requirement for HSPGs. To assess the role of receptors, we used either receptor-associated protein, the 39-kDa inhibitor of ligand binding to the LDL receptor-related protein and the very low density lipoprotein (VLDL) receptor, or specific receptor antibodies. Receptor-associated protein reduced125I-LPL and LPL activity transfer across the monolayers. When the basolateral surface of the cells was treated with antibodies, only anti-VLDL receptor antibodies inhibited transcytosis. Moreover, overexpression of the VLDL receptor using adenoviral-mediated gene transfer increased LPL transcytosis. Thus, movement of active LPL across endothelial cells involves both HSPGs and VLDL receptor. Lipoprotein lipase (LPL), the major enzyme responsible for the hydrolysis of circulating lipoprotein triglyceride molecules, is synthesized in myocytes and adipocytes but functions while bound to heparan sulfate proteoglycans (HSPGs) on the luminal surface of vascular endothelial cells. This requires transfer of LPL from the abluminal side to the luminal side of endothelial cells. Studies were performed to investigate the mechanisms of LPL transcytosis using cultured monolayers of bovine aortic endothelial cells. We tested whether HSPGs and members of the low density lipoprotein (LDL) receptor superfamily were involved in transfer of LPL from the basolateral to the apical side of cultured endothelial cells. Heparinase/heparinitase treatment of the basolateral cell surface or addition of heparin to the basolateral medium decreased the movement of LPL. This suggested a requirement for HSPGs. To assess the role of receptors, we used either receptor-associated protein, the 39-kDa inhibitor of ligand binding to the LDL receptor-related protein and the very low density lipoprotein (VLDL) receptor, or specific receptor antibodies. Receptor-associated protein reduced125I-LPL and LPL activity transfer across the monolayers. When the basolateral surface of the cells was treated with antibodies, only anti-VLDL receptor antibodies inhibited transcytosis. Moreover, overexpression of the VLDL receptor using adenoviral-mediated gene transfer increased LPL transcytosis. Thus, movement of active LPL across endothelial cells involves both HSPGs and VLDL receptor. lipoprotein lipase low density lipoprotein LDL receptor-related protein very low density lipoprotein VLDL receptor heparan sulfate proteoglycan receptor-associated protein bovine aortic endothelial cell Dulbecco's modified Eagle's medium bovine serum albumin human VLDLr expressing adenovirus β-galactosidase expressing adenovirus interleukin-8 Lipoprotein lipase (LPL)1 is a 120-kDa dimeric protein that associates with the luminal surface of endothelial cells in multiple organs but especially in cardiac and skeletal muscle and in adipose tissue (1Zechner R. Curr. Opin. Lipidol. 1997; 8: 77-88Crossref PubMed Scopus (143) Google Scholar). This enzyme hydrolyzes the triglyceride in circulating lipoproteins such as chylomicrons and VLDL and produces free fatty acids that are used for metabolic energy or for fat storage. Endothelial cells do not synthesize LPL; rather myocytes and adipocytes produce it. Thus, it is a protein that requires transcytosis across the endothelial cell barrier, in this case from the interstitial fluid to the luminal side of the cells. There are several possible ways that LPL could cross the endothelial barrier. Nonspecific transport of molecules across endothelial monolayers occurs either via paracellular routes between the cells or via vesicular transit through cells (2Lum H. Malik A.B. Can. J. Physiol. Pharmacol. 1996; 74: 787-800PubMed Google Scholar). Alternatively, a specific transcytosis pathway could exist which requires LPL to associate with a cell surface receptor and then transports LPL through the cells. This process would be analogous to that which transfers IgA across epithelial cells (3Mostov K. Semin. Cell Biol. 1991; 2: 411-418PubMed Google Scholar). The first step in a specific LPL transcytosis pathway would involve LPL interaction with the basolateral side of endothelial cells. LPL binds to a number of cell surface molecules including heparan sulfate proteoglycans (HSPGs) and members of the LDL receptor family (4Goldberg I.J. J. Lipid Res. 1996; 37: 693-707Abstract Full Text PDF PubMed Google Scholar). In bovine endothelial cells the most highly expressed of these receptors is the VLDL receptor (VLDLr) (5Magrane J. Reina M. Pagan R. Luna A. Casaroli-Marano R.P. Angelin B. Gafvels M. Vilaro S. J. Lipid Res. 1998; 39: 2172-2181Abstract Full Text Full Text PDF PubMed Google Scholar). A previous study suggested that HSPGs are required for LPL transcytosis (6Saxena U. Klein M.G. Goldberg I.J. Proc. Natl. Acad. Sci. U. S. A. 1991; 88: 2254-2258Crossref PubMed Scopus (79) Google Scholar). It is, however, unclear whether HSPGs are sufficient for transport or whether HSPGs must operate in concert with receptors. The binding of LPL to several members of the LDL receptor family leads to uptake and degradation of LPL by cells. There are no data on whether these receptors participate in transendothelial movement of LPL or other ligands. In this report, we present data showing that LPL transcytosis across endothelial monolayers requires both HSPGs and the VLDLr. LPL transcytosis was diminished by removal of HSPGs and inhibition of receptors by RAP, a 39-kDa protein that was copurified with the LDL receptor-related protein (LRP) (7Williams S.E. Kounnas M.Z. Argraves K.M. Argraves W.S. Strickland D.K. Ann. N. Y. Acad. Sci. 1994; 737: 1-13Crossref PubMed Scopus (40) Google Scholar). This protein binds to members of the LDL receptor family and inhibits ligand binding and uptake by those receptors (8Williams S.E. Ashcom J.D. Argraves W.S. Strickland D.K. J. Biol. Chem. 1992; 267: 9035-9040Abstract Full Text PDF PubMed Google Scholar, 9Herz J. Goldstein J.L. Strickland D.K. Ho Y.K. Brown M.S. J. Biol. Chem. 1991; 266: 21232-21238Abstract Full Text PDF PubMed Google Scholar). Furthermore, antibodies against the VLDLr blocked LPL translocation and increased expression of this receptor-increased transcytosis. Thus, LPL requires both HSPGs and receptors for translocation across endothelial cells. LPL was purified from unpasteurized bovine milk according to the method of Socorroet al. (10Socorro L. Green C.C. Jackson R.L. Prep. Biochem. 1985; 15: 133-143Crossref PubMed Scopus (26) Google Scholar) with modifications as described by Saxenaet al. (11Saxena U. Witte L.D. Goldberg I.J. J. Biol. Chem. 1989; 264: 4349-4355Abstract Full Text PDF PubMed Google Scholar). 300–500 μg/ml purified enzyme was stored at −70 °C. Enzyme activity was assayed with a glycerol-containing triolein emulsion as described previously (11Saxena U. Witte L.D. Goldberg I.J. J. Biol. Chem. 1989; 264: 4349-4355Abstract Full Text PDF PubMed Google Scholar). The purified enzyme had a specific activity of 40–50 mmol of oleic acid released/h/mg of enzyme at 37 °C. LPL was radioiodinated enzymatically with glucose oxidase and lactoperoxidase (12Pillarisetti S. Methods Mol. Biol. 1999; 109: 267-278PubMed Google Scholar). Radioiodinated LPL was purified by heparin-agarose (Bio-Rad) affinity chromatography and stored at −70 °C. Typical specific activity of the preparation was 1,000–2,000 cpm/ng, and >90% of the radioactivity was precipitated with trichloroacetic acid. 125I-LPL was purified by Sephadex G-25 gel filtration (PD-10, Amersham Pharmacia Biotech) prior to use to remove degradation products. Heat-inactivated LPL was prepared by heating LPL for 1 h at 52 °C. Primary cultures of bovine aortic endothelial cells (BAECs) were established as reported (13Cornicelli J.A. Witte L.D. Goodman D.S. Arteriosclerosis. 1983; 3: 560-567Crossref PubMed Google Scholar) and were grown in DMEM containing 10% fetal bovine serum (Gemini Bioproducts Inc., Calabasas, CA), 1% (v/v) penicillin and streptomycin solution, and 1% (v/v) glutamine solution (both from Life Technologies, Inc.). Polarized BAEC monolayers were grown on gelatin and fibronectin-coated polyethylene terephthalate 10-mm filters (pore diameter, 3.0 μm) (Becton Dickinson Labware, Franklin Lakes, NJ). This allowed access to both the basolateral side of the cells adjacent to the lower chamber and the apical cell surface in contact with medium in the upper chamber (14Stins M.F. Nemani P.V. Wass C. Kim K.S. Infect. Immun. 1999; 67: 5522-5525Crossref PubMed Google Scholar). Approximately 4–5 × 104cells were seeded onto the filters. Experiments on nonviral infected cells were conducted 5–6 days after seeding the endothelial cells. The media in the upper chamber (0.5 ml) and lower chamber (1 ml) were changed every other day. Movement of both [3H]dextran (70 kDa, American Radiolabeled Chemicals, St. Louis, MO) and LDL was routinely assessed to verify that the cell was transport of 125I-LPL across the monolayers was after 1 μg/ml LPL to DMEM and in the basolateral In the basolateral side of the cells was with Inc., or μg/ml 1 the medium was and the cells were prior to that in the upper chamber was by of medium from the apical side of the cells at and The were not to of the monolayers. were performed using the of the the cells were and 125I-LPL with luminal and basolateral was by the addition of containing heparin at for to upper and lower was then by the cells in and by the In other heparin was to the basolateral chamber with the and the of 125I-LPL protein in the upper chamber was The protein was routinely with 10% trichloroacetic in 10% of the in the were not precipitated To study the transport of LPL μg/ml LPL purified from bovine milk as described was to the basolateral medium LPL activity a 37 and is in this of LPL was required to transport of In the activity was at In LPL was with μg/ml or heparin or after a of the cells with VLDLr antibodies. of medium from the upper chamber was at and and at −70 °C. The were and assayed these we used a LPL activity described by B. Biochem. Physiol. B. Biochem. Mol. Biol. 1998; PubMed Scopus Google and were performed in Pharmacia Biotech) was with of to produce a 10% To the the emulsion was with of Amersham Pharmacia of cell medium was for 1 h at with of the of serum as a of of fatty bovine serum of and of The was by the addition of of and of a solution containing a at × of the upper was to containing of and 1 of of A was performed as and the upper was by of The were then in the The radioactivity in of the was using of fluid in a was as a protein with in expression human (7Williams S.E. Kounnas M.Z. Argraves K.M. Argraves W.S. Strickland D.K. Ann. N. Y. Acad. Sci. 1994; 737: 1-13Crossref PubMed Scopus (40) Google Scholar). against LPL and antibodies against the LDL receptor, and were described previously D.K. Ashcom J.D. S. M. Argraves W.S. J. Biol. Chem. Full Text PDF PubMed Google Scholar, S. Y. J. Proc. Natl. Acad. Sci. U. S. A. 1992; PubMed Scopus Google Scholar, J.D. Strickland D.K. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, J.D. S. M. Strickland D.K. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar). expression of VLDLr in were infected with human VLDL receptor and adenovirus the cells were using these cells were conducted h after The of the endothelial cell monolayers was using and transport as described previously (6Saxena U. Klein M.G. Goldberg I.J. Proc. Natl. Acad. Sci. U. S. A. 1991; 88: 2254-2258Crossref PubMed Scopus (79) Google Scholar). Cell from and were prepared as described K.M. Gafvels M. Strickland D.K. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). on 10-mm filters were in of solution containing 1% 1 μg/ml and μg/ml The cell was with a and then at for The of in was and ligand was on gel and to a The was with in containing and for 1 h at The was then with 1 in for 1 h at and in containing The was then with a (Bio-Rad) for 1 h at °C. the were using the Pharmacia We first assessed of LPL to the basolateral side of the of LPL that the in in the medium on the basolateral side of the cells to 125I-LPL in the upper the 125I-LPL 1 a lower of the LPL was that 1 μg/ml was used for To the of LPL we used LPL to 125I-LPL The addition of μg/ml LPL to the lower chamber in the of 1 μg/ml 125I-LPL decreased the of 125I-LPL in the upper chamber to of 1 The the cells and from the apical surface are in and was the cells on the cell of 125I-LPL in the lower chamber to LPL in the cells and on the apical In the of of both the and cell LPL were decreased by uptake and transfer to the apical surface were but not the role of HSPGs and members of the LDL receptor family as molecules is only L. M.G. Goldberg I.J. J. Lipid Res. 1996; 37: Full Text PDF PubMed Google Scholar, Biochem. J. 1985; PubMed Scopus Google Scholar) performed with to role as transcytosis members of the this family of receptors in concert with HSPGs L. M.G. Goldberg I.J. J. Lipid Res. 1996; 37: Full Text PDF PubMed Google Scholar, Biochem. J. 1985; PubMed Scopus Google we tested whether LPL movement across monolayers requires with HSPGs. 125I-LPL translocation across BAEC monolayers was in the of in heparin at in the upper chamber after h by was with a of heparin To whether degradation LPL HSPGs on the basolateral side of the cells were by the cells with The in that treatment of the basolateral side of the cells 125I-LPL movement across the a Thus, LPL movement across the monolayers required with proteoglycans on the basolateral side of the endothelial cells. a of the LPL was not by and This suggested that the LPL was transport only of which was by LPL and inhibition of We whether HSPGs are sufficient for LPL transcytosis or whether as molecules for transcytosis receptors. To whether or other members of this family are involved in LPL 1 was to the lower chamber of monolayers that had for 1 h with The had to the lower chamber in to produce a of of LPL to that LPL is In a movement from the lower to the upper chamber The μg/ml as and μg/ml as the as it The transport with μg/ml was on the Moreover, the of inhibition at at inhibition was using the of We tested whether the and inhibition were in heparin was to the cells the inhibition of LPL transport was to that using only The addition of LPL not to inhibition of transport that with This that the RAP, and were the To assess whether movement of a 1 μg/ml was to the basolateral side of the and movement the h was movement to the upper chamber is in this was not by heparin or the addition of Movement of LPL from the apical to the basolateral side of the monolayers was in the and of and using cells that were treated with LPL movement was in this the transport across the cells with that in had for movement in the both and removal of from HSPGs decreased the of 125I-LPL in the medium on the basolateral side of the cells. The of LPL the cells and on the apical surface was after a the of 125I-LPL In the in the that a from to 125I-LPL was from to protein by the cell monolayers with heparin or heparin RAP, decreased both cell surface and LPL. was RAP, it decreased both receptor and uptake of to heparin to no in or cell and heparin decreased both LPL in the upper chamber and in the it suggested that the LPL movement was via To investigate the of and heparin on the of LPL activity on the apical side of the endothelial cell μg/ml purified LPL was to the lower chamber in the or of μg/ml RAP, or to monolayers in which the basolateral side of the cells was treated with μg/ml in treatment of the cells with and addition of with LPL or heparin the of LPL activity in the upper 1 of the decreased LPL activity transport by >90% with cells. A inhibition of LPL activity movement through the endothelial was at h not and heparin decreased the of LPL activity from the lower to the upper This of and heparin on LPL activity transcytosis was that possible for the of on LPL activity 125I-LPL was that LPL protein was across the monolayers by a It be that these LPL would to and the data using this would assess both active and LPL. To whether LPL was in a to that of active dimeric was previously and of LPL that from heparin at a lower and is to be Biochem. J. 1985; PubMed Scopus Google Scholar). preparation was assessed by gel and of however, of this preparation from heparin affinity gel with This is in to in which of the 125I-LPL at this in 125I-LPL was from the lower to the upper chamber as as 125I-LPL Moreover, this transfer was by either the addition of with the or of the endothelial cells with 125I-LPL the at a via a against LPL or members of the LDL receptor family were to the lower chamber prior to the addition of of antibodies were sufficient to the receptors Strickland D.K. J. Biol. Chem. Full Text PDF PubMed Google Scholar) or to to LPL in and antibodies inhibited transcytosis by these data are in the and In antibodies to the LDL receptor and had no The treatment was used to assess the role of the VLDLr in transport of active LPL antibodies decreased LPL movement to the apical side of the cells by This inhibition was to that with and not on inhibition of inhibition of the VLDLr decreased LPL transport across the cultured endothelial cells. We tested whether VLDLr expression LPL transcytosis. were infected with either or and the expressed VLDLr was by ligand of In and infected cells a for a protein which to the VLDLr was The of this was using antibodies. cells had human VLDLr in a lower of the VLDLr human cells (5Magrane J. Reina M. Pagan R. Luna A. Casaroli-Marano R.P. Angelin B. Gafvels M. Vilaro S. J. Lipid Res. 1998; 39: 2172-2181Abstract Full Text Full Text PDF PubMed Google Scholar). other were The with the protein VLDLr expression increased LPL A transcytosis across and infected is in B. infected with are cells are in and cells are VLDLr overexpression transport in after blocked the increased transcytosis to and the transport to that of cells. data that LPL transport across endothelial cells be by the VLDLr. and the of VLDLr overexpression on apical cell surface and infected with had and cell surface 125I-LPL 125I-LPL on the apical surface was and that the cells was increased This was blocked by Thus, VLDLr overexpression increased uptake and transfer of LPL across the that LPL transport from the basolateral to the apical side of cultured endothelial cell monolayers is by LPL binding to HSPGs and interaction with the VLDLr. previous (6Saxena U. Klein M.G. Goldberg I.J. Proc. Natl. Acad. Sci. U. S. A. 1991; 88: 2254-2258Crossref PubMed Scopus (79) Google Scholar) and the of HSPGs in the LPL transcytosis for that the role of binding is to the LPL on the cell surface and the of binding to cell surface receptors. data suggested that receptors, especially the were involved in LPL movement across the monolayers. of the monolayers with decreased the transfer of LPL from the abluminal to luminal side of the cells. of LPL activity to the apical side of the cells was inhibited to by against the VLDLr decreased LPL of the VLDLr in endothelial cells increased LPL of LPL 125I-LPL transport across BAEC monolayers. we that the of not Furthermore, RAP, or VLDLr antibodies were used to 125I-LPL the was no in most of the for the of these is the that purified LPL to a 37 J. N. and Biochem. Scholar). that assessed the transfer of both active and LPL. this both the and we were to for this of this Studies were performed using and assayed movement of LPL Heat-inactivated LPL across the and transport was not decreased by In LPL activity transcytosis was blocked by RAP, or VLDLr antibodies. and active dimeric LPL in LPL transcytosis occurs via a and 125I-LPL transfer through paracellular or HSPGs and the VLDLr and cell surface LPL. 125I-LPL was on the apical surface of the cells. This was not endothelial cells and LPL U. Klein M.G. Goldberg I.J. J. Biol. Chem. Full Text PDF PubMed Google and of the LPL for of the that decreased the of 125I-LPL in the upper chamber medium decreased apical cell surface and RAP, and antibodies. movement of LPL to both the surface and the medium is to be via the We that LPL across the cell only it is via the VLDLr Endothelial other very LPL U. Klein M.G. Goldberg I.J. J. Biol. Chem. Full Text PDF PubMed Google Scholar). A was by Y. PubMed Scopus Google Scholar) and Argraves al. K.M. Gafvels M. Strickland D.K. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). We tested the used in these and that cells J. S. J. H. C. M. A. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar) of cell surface 125I-LPL was by the cells. the VLDLr LPL degradation in other receptors in this family do not to degradation of ligands. It that uptake of leads to of this S. M.F. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). is a protein that with Thus, it is that the cell whether these receptors participate in protein or In with this it reported that is responsible for transcytosis of by cells grown on filters M. L. A. Brown J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar) this receptor leads to degradation of ligands. is protein that binds to both proteoglycans and receptors Biochem. Pharmacol. 1997; PubMed Scopus Google Scholar, Curr. Biol. 1994; Full Text Full Text PDF PubMed Scopus Google Scholar). this protein the via the and then in the A study J. S. J. H. C. M. A. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar) that of blocked the in the it was to to a step required for interaction with the receptor. that a process is required for LPL. in cultured HSPGs and VLDLr as of a LPL transport must whether these are of for LPL and adipose tissue are the most of hydrolysis of lipoprotein are of VLDLr expression S. Y. J. Proc. Natl. Acad. Sci. U. S. A. 1992; PubMed Scopus Google Scholar). This receptor is highly expressed in endothelial cells K. Biol. 1996; PubMed Scopus Google Scholar) is in the and cells to transport LPL. Moreover, the VLDLr is by and and expression in LPL activity in those S. A. Res. 1997; PubMed Scopus Google Scholar). The that inhibits the transcytosis of LPL to of is with and several in of in increased circulating triglyceride increased of lipoproteins were only in LDL receptor S. J. 1994; 264: PubMed Scopus Google Scholar). was with in the of circulating lipoproteins R.L. R. J. J. 1998; PubMed Scopus Google Scholar). In in the overexpression LPL activity A. A. J. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). other uptake via In not we that inhibits LPL binding to the luminal side of endothelial cells inhibits hydrolysis of emulsion or VLDL by A. A. J. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). We that inhibition of LPL transcytosis is the of of data is the of lipoprotein in VLDLr Brown M.S. Goldstein J.L. J. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). however, are not expressing LPL in fat a lipoprotein but with are S. H. R. J.L. Proc. Natl. Acad. Sci. U. S. A. 1997; PubMed Scopus Google Scholar). possible for the of in the VLDLr is that LPL transcytosis is not It be that LPL activity in is in Y. J. R. Goldberg I.J. J. Lipid Res. 1999; Full Text Full Text PDF PubMed Google a of LPL activity be sufficient for In of this is a showing that in both VLDLr and the LDL receptor are on a fat B. K. J. Lipid Res. Full Text Full Text PDF PubMed Google Scholar). Thus, lipoprotein through the be by VLDLr In previous from this we LPL interaction with the surface of endothelial cells. a number of we reported that LPL binds to a heparan sulfate proteoglycan on the luminal surface of endothelial cells U. Klein M.G. Goldberg I.J. J. Biol. Chem. 1991; 266: Full Text PDF PubMed Google Scholar). This cell surface proteoglycan is LPL binding is by the on the and a highly is required for LPL binding N. Goldberg I.J. B. J. Biol. Chem. 1994; Full Text PDF PubMed Google Scholar). We reported that LPL bound to a protein that was not a proteoglycan Klein M.G. Goldberg I.J. J. Biol. Chem. 1992; 267: Full Text PDF PubMed Google Scholar). A protein was and and was of Goldberg I.J. J. Biol. Chem. 1994; Full Text PDF PubMed Google this of associates with LPL and heparin A. Goldberg I.J. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). The VLDLr a of (5Magrane J. Reina M. Pagan R. Luna A. Casaroli-Marano R.P. Angelin B. Gafvels M. Vilaro S. J. Lipid Res. 1998; 39: 2172-2181Abstract Full Text Full Text PDF PubMed Google Scholar). It is that to by ligand and LPL affinity chromatography the VLDLr of to the In we that LPL transcytosis across endothelial cells is inhibited by LPL interaction with HSPGs and by the VLDLr. of the VLDLr this In a pathway that for movement of LPL molecules and is not to or data that HSPGs participate with members of the LDL receptor family to and transfer from the interstitial to the This process be for the transport pathway of active LPL. are to be involved in the of a number of other that must transfer from cells to the We are to suggested a number of the from which these and data on the in of on LPL and lipoprotein