Apolipoprotein A-V association with intracellular lipid droplets

Apolipoprotein A-V (apoA-V) plays a key role in the regulation of triglyceride (TG) metabolism. Given the very low concentration of apoA-V in plasma, we hypothesized that apoA-V may influence plasma TG levels by affecting the assembly and/or secretion of apoB-containing lipoproteins. When apoA-V was overexpressed in cultured Hep3B cells, neither the amount of apoB secreted nor the density distribution of apoB-containing lipoproteins was affected. Fluorescence microscopy and cell lysate immunoprecipitation studies revealed that apoA-V is not associated with apoB intracellularly, yet immunoprecipitation of apoA-V from the cell culture medium resulted in coprecipitation of apoB. These data suggest that the apoA-V association with apoB-containing lipoproteins is a postsecretory event. Confocal fluorescence microscopy revealed the presence of apoA-V in distinct cellular structures. Based on Nile Red staining, we identified these structures to be intracellular lipid droplets. These data suggest that apoA-V has a unique association with cellular lipids and, therefore, may be involved in the storage or mobilization of intracellular lipids. Apolipoprotein A-V (apoA-V) plays a key role in the regulation of triglyceride (TG) metabolism. Given the very low concentration of apoA-V in plasma, we hypothesized that apoA-V may influence plasma TG levels by affecting the assembly and/or secretion of apoB-containing lipoproteins. When apoA-V was overexpressed in cultured Hep3B cells, neither the amount of apoB secreted nor the density distribution of apoB-containing lipoproteins was affected. Fluorescence microscopy and cell lysate immunoprecipitation studies revealed that apoA-V is not associated with apoB intracellularly, yet immunoprecipitation of apoA-V from the cell culture medium resulted in coprecipitation of apoB. These data suggest that the apoA-V association with apoB-containing lipoproteins is a postsecretory event. Confocal fluorescence microscopy revealed the presence of apoA-V in distinct cellular structures. Based on Nile Red staining, we identified these structures to be intracellular lipid droplets. These data suggest that apoA-V has a unique association with cellular lipids and, therefore, may be involved in the storage or mobilization of intracellular lipids. apolipoprotein A-V green fluorescent protein methionine/cysteine oleic acid triglyceride Epidemiological studies have revealed that increased plasma triglyceride (TG) represents an independent risk factor for coronary heart disease (1.Cullen P. Evidence that triglycerides are an independent coronary heart disease risk factor.Am. J. Cardiol. 2000; 86: 943-949Abstract Full Text Full Text PDF PubMed Scopus (374) Google Scholar). Apolipoprotein A-V (apoA-V), a protein expressed solely in the liver, was shown to be involved in TG metabolism in rodents (2.Pennacchio L.A. Olivier M. Hubacek J.A. Cohen J.C. Cox D.R. Fruchart J.C. Krauss R.M. Rubin E.M. An apolipoprotein influencing triglycerides in humans and mice revealed by comparative sequencing.Science. 2001; 294: 169-173Crossref PubMed Scopus (801) Google Scholar, 3.van der Vliet H.N. Sammels M.G. Leegwater A.C. Levels J.H. Reitsma P.H. Boers W. Chamuleau R.A. Apolipoprotein A-V: a novel apolipoprotein associated with an early phase of liver regeneration.J. Biol. Chem. 2001; 276: 44512-44520Abstract Full Text Full Text PDF PubMed Scopus (257) Google Scholar). ApoA-V is synthesized with a cleavable signal peptide, suggesting that it will be directed toward a secretory pathway. The concentration of apoA-V in human plasma, however, is extremely low (24–406 ng/ml) (4.O'Brien P.J. Alborn W.E. Sloan J.H. Ulmer M. Boodhoo A. Knierman M.D. Schultze A.E. Konrad R.J. The novel apolipoprotein A5 is present in human serum, is associated with VLDL, HDL, and chylomicrons, and circulates at very low concentrations compared with other apolipoproteins.Clin. Chem. 2005; 51: 351-359Crossref PubMed Scopus (182) Google Scholar) compared with other apolipoproteins, such as apoA-I (∼1 mg/ml) or apoC-III (∼0.1 mg/ml), bringing into question the functional role of plasma apoA-V. Compared with control littermates, the concentration of TG was increased in homozygous apoA-V knockout mice and decreased in human apoA-V transgenic mice (2.Pennacchio L.A. Olivier M. Hubacek J.A. Cohen J.C. Cox D.R. Fruchart J.C. Krauss R.M. Rubin E.M. An apolipoprotein influencing triglycerides in humans and mice revealed by comparative sequencing.Science. 2001; 294: 169-173Crossref PubMed Scopus (801) Google Scholar). The TG-lowering effect of apoA-V in mice was confirmed by van der Vliet et al. (5.van der Vliet H.N. Schaap F.G. Levels J.H. Ottenhoff R. Looije N. Wesseling J.G. Groen A.K. Chamuleau R.A. Adenoviral overexpression of apolipoprotein A-V reduces serum levels of triglycerides and cholesterol in mice.Biochem. Biophys. Res. Commun. 2002; 295: 1156-1159Crossref PubMed Scopus (153) Google Scholar) using adenoviral overexpression of mouse apoA-V. Population studies of APOAV single nucleotide polymorphisms (2.Pennacchio L.A. Olivier M. Hubacek J.A. Cohen J.C. Cox D.R. Fruchart J.C. Krauss R.M. Rubin E.M. An apolipoprotein influencing triglycerides in humans and mice revealed by comparative sequencing.Science. 2001; 294: 169-173Crossref PubMed Scopus (801) Google Scholar, 6.Vrablik M. Horinek A. Ceska R. Adamkova V. Poledne R. Hubacek J.A. Ser19→Trp polymorphism within the apolipoprotein AV gene in hypertriglyceridaemic people.J. Med. Genet. 2003; 40: e105Crossref PubMed Scopus (27) Google Scholar, 7.Pennacchio L.A. Olivier M. Hubacek J.A. Krauss R.M. Rubin E.M. Cohen J.C. Two independent apolipoprotein A5 haplotypes influence human plasma triglyceride levels.Hum. Mol. Genet. 2002; 11: 3031-3038Crossref PubMed Google Scholar, 8.Talmud P.J. Palmen J. Putt W. Lins L. Humphries S.E. Determination of the functionality of common APOA5 polymorphisms.J. Biol. Chem. 2005; 280: 28215-28220Abstract Full Text Full Text PDF PubMed Scopus (107) Google Scholar) and truncated apoA-V variants (9.Marcais C. Verges B. Charriere S. Pruneta V. Merlin M. Billon S. Perrot L. Drai J. Sassolas A. Pennacchio L.A. et al.Apoa5 Q139X truncation predisposes to late-onset hyperchylomicronemia due to lipoprotein lipase impairment.J. Clin. Invest. 2005; 115: 2862-2869Crossref PubMed Scopus (140) Google Scholar, 10.Oliva C.P. Pisciotta L. Volti G.Li Sambataro M.P. Cantafora A. Bellocchio A. Catapano A. Tarugi P. Bertolini S. Calandra S. Inherited apolipoprotein A-V deficiency in severe hypertriglyceridemia.Arterioscler. Thromb. Vasc. Biol. 2005; 25: 411-417Crossref PubMed Scopus (168) Google Scholar) have provided support for the hypothesis that apoA-V influences human plasma TG levels. The mechanism whereby apoA-V decreases TG is not completely understood, but it has been suggested that apoA-V may enhance LPL activity, either directly or indirectly (11.Fruchart-Najib J. Bauge E. Niculescu L.S. Pham T. Thomas B. Rommens C. Majd Z. Brewer B. Pennacchio L.A. Fruchart J.C. Mechanism of triglyceride lowering in mice expressing human apolipoprotein A5.Biochem. Biophys. Res. Commun. 2004; 319: 397-404Crossref PubMed Scopus (177) Google Scholar, 12.Schaap F.G. Rensen P.C. Voshol P.J. Vrins C. van der Vliet H.N. Chamuleau R.A. Havekes L.M. Groen A.K. van Dijk K.W. ApoAV reduces plasma triglycerides by inhibiting very low density lipoprotein-triglyceride (VLDL-TG) production and stimulating lipoprotein lipase-mediated VLDL-TG hydrolysis.J. Biol. Chem. 2004; 279: 27941-27947Abstract Full Text Full Text PDF PubMed Scopus (259) Google Scholar, 13.Merkel M. Loeffler B. Kluger M. Fabig N. Geppert G. Pennacchio L.A. Laatsch A. Heeren J. Apolipoprotein AV accelerates plasma hydrolysis of triglyceride-rich lipoproteins by interaction with proteoglycan-bound lipoprotein lipase.J. Biol. Chem. 2005; 280: 21553-21560Abstract Full Text Full Text PDF PubMed Scopus (249) Google Scholar). Considering the extremely low concentration of apoA-V in the circulation, however, it is conceivable that the metabolic role for apoA-V is intracellular rather than extracellular. Using apoA-V transfected COS-1 cells, Weinberg et al. (14.Weinberg R.B. Cook V.R. Beckstead J.A. Martin D.D. Gallagher J.W. Shelness G.S. Ryan R.O. Structure and interfacial properties of human apolipoprotein A-V.J. Biol. Chem. 2003; 278: 34438-34444Abstract Full Text Full Text PDF PubMed Scopus (136) Google Scholar) found that, compared with human serum albumin or apoB-6.6, apoA-V is largely retained in the cell. An intracellular function for apoA-V was suggested by Schaap et al. (12.Schaap F.G. Rensen P.C. Voshol P.J. Vrins C. van der Vliet H.N. Chamuleau R.A. Havekes L.M. Groen A.K. van Dijk K.W. ApoAV reduces plasma triglycerides by inhibiting very low density lipoprotein-triglyceride (VLDL-TG) production and stimulating lipoprotein lipase-mediated VLDL-TG hydrolysis.J. Biol. Chem. 2004; 279: 27941-27947Abstract Full Text Full Text PDF PubMed Scopus (259) Google Scholar), who found that adenovirus-mediated apoA-V expression in mice decreases VLDL-TG production rate in a dose-dependent manner yet has no effect on VLDL particle number, suggesting that apoA-V impairs the lipidation of apoB but does not impair its secretion. Finally, the observation by van der Vliet et al. (3.van der Vliet H.N. Sammels M.G. Leegwater A.C. Levels J.H. Reitsma P.H. Boers W. Chamuleau R.A. Apolipoprotein A-V: a novel apolipoprotein associated with an early phase of liver regeneration.J. Biol. Chem. 2001; 276: 44512-44520Abstract Full Text Full Text PDF PubMed Scopus (257) Google Scholar) that apoA-V mRNA is upregulated during liver regeneration suggests that apoA-V serves a function in hepatocyte proliferation. Using the human liver cell line Hep3B, we examined the effect of apoA-V expression on apoB-100 secretion and lipidation. Surprisingly, we found that apoA-V does not colocalize with apoB intracellularly but, rather, can be found in association with cytosolic lipid droplets. [35S]methionine/cysteine (Met/Cys) was purchased from GE Healthcare. Oleic acid (OA), albumin, monoclonal anti-FLAG M2 antibody, and anti-FLAG M2 affinity gel were from Sigma. MEM, DMEM without Met and Cys, sodium pyruvate solution, MEM nonessential amino acid solution, FBS, horse serum, G418, and trypsin-EDTA were purchased from Gibco. Fluorescence-labeled goat anti-mouse Alexa Fluor 594, Nile Red, and 4′,6-diamino-phenylindole (DAPI) were from Molecular Probes. Antibodies used included polyclonal goat anti-apoA-V(15), monoclonal mouse anti-apoE 1D7 (a gift from Dr. Karl Weisgraber), anti-human apoB monoclonal antibody, 1D1, which recognizes only human apoB (University of Ottawa Heart Institute), polyclonal goat anti-apoB (International Immunology Corp.), and monoclonal mouse anti-apoA-I (Intracel). The human hepatocarcinoma cell line Hep3B (American Type Culture Collection) was cultured in MEM containing 10% FBS, 1 mM sodium pyruvate, and 100 μM nonessential amino acids. Rat hepatoma McA-RH7777-A18 cells stably transfected with human apoB-100 (kindly provided by Dr. Zemin Yao) were cultured in DMEM containing 10% FBS, 10% horse serum, and 200 μg/ml G418. Cells were passaged every 4 days. Cells were transfected using Lipofectamine 2000 (Invitrogen). To obtain a FLAG-tagged apoA-V (apoA-V-FLAG) expression vector and apoA-V-green fluorescent protein (GFP) fusion protein expression vector, PCR was carried out using a plasmid harboring the entire apoA-V coding region (a gift from Dr. Len Pennacchio). The amplification product for the FLAG tag was cloned into pFLAG-CMV-5.1 (Sigma) and that of GFP was cloned into pEGFP-N1 (Clontech) via HindIII and BamHI sites, respectively. Both tags were appended to the C terminus of the protein. Collected cells were washed with cold phosphate-buffered saline and subsequently lysed in a nondenaturing lysis buffer described by Beckstead et al. (15.Beckstead J.A. Oda M.N. Martin D.D. Forte T.M. Bielicki J.K. Berger T. Luty R. Kay C.M. Ryan R.O. Structure-function studies of human apolipoprotein A-V: a regulator of plasma lipid homeostasis.Biochemistry. 2003; 42: 9416-9423Crossref PubMed Scopus (71) Google Scholar). Immunoprecipitation was carried out according to Wu et al. (16.Wu X. Zhou M. Huang L-S. Wetterau J. Ginsberg H.N. Demonstration of a physical interaction between microsomal triglyceride transfer protein and apolipoprotein B during the assembly of apoB-containing lipoproteins.J. Biol. Chem. 1996; 271: 10277-10281Abstract Full Text Full Text PDF PubMed Scopus (124) Google Scholar). Protein samples were electrophoresed and immunoblots processed as described previously (17.Dixon J.L. Furukawa S. Ginsberg H.N. Oleate stimulates secretion of apolipoprotein B-containing lipoproteins from Hep G2 cells by inhibiting early intracellular degradation of apolipoprotein B.J. Biol. Chem. 1991; 266: 5080-5086Abstract Full Text PDF PubMed Google Scholar). Cells were incubated in Met/Cys-deficient DMEM for 1 h, then pulsed with [35S]Met/Cys (200 μCi/ml) in Met/Cys-free DMEM containing 10% FBS and 0.8 mM OA for 20 min followed by a 3 h chase. Conditioned medium was collected and subjected to cumulative rate flotation centrifugation (18.Tran K. Thorne-Tjomsland G. DeLong C.J. Cui Z. Shan J. Burton L. Jamieson J.C. Yao Z. Intracellular assembly of very low density lipoproteins containing apolipoprotein B100 in rat hepatoma McA-RH7777 cells.J. Biol. Chem. 2002; 277: 31187-31200Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar). One milliliter fractions were collected, immunoprecipitated, and analyzed by SDS-PAGE followed by autoradiography. McA-RH7777-A18 cells were grown on poly-l-lysine coverslips (BD Biosciences). After transfection with apoA-V-GFP or apoA-V-FLAG, cells were transferred to growth medium supplemented with 0.8 mM OA for 6 h. For immunocytochemistry, cells were fixed with 4% paraformaldehyde in PBS and processed as described (18.Tran K. Thorne-Tjomsland G. DeLong C.J. Cui Z. Shan J. Burton L. Jamieson J.C. Yao Z. Intracellular assembly of very low density lipoproteins containing apolipoprotein B100 in rat hepatoma McA-RH7777 cells.J. Biol. Chem. 2002; 277: 31187-31200Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar). Anti-human apoB monoclonal antibody 1D1 was diluted 1:500 in blocking solution and incubated with the cells for 1 h. Cells were washed with PBS and incubated with Alexa Fluor 594-labeled goat anti-mouse IgG. For lipid droplet staining, a Nile Red-saturated (19.Greenspan P. Mayer E.P. Fowler S.D. Nile Red: a selective fluorescent stain for intracellular lipid droplets.J. Cell Biol. 1985; 100: 965-973Crossref PubMed Scopus (1858) Google Scholar) acetone solution was diluted 1:100,000 in PBS and incubated with cells for 15 min. Images were captured by a LSM 510 Meta ultraviolet/visible confocal microscope. Hep3B cells were transfected with a control empty vector or apoA-V plasmid construct. Immunoblot experiments (Fig. 1) with the indicated antibodies were performed to determine the relative distribution of apolipoprotein in cell lysates versus conditioned medium. In Hep3B cells transfected with an empty vector, apoA-V could not be detected, consistent with the very low concentration of this protein reported by others (4.O'Brien P.J. Alborn W.E. Sloan J.H. Ulmer M. Boodhoo A. Knierman M.D. Schultze A.E. Konrad R.J. The novel apolipoprotein A5 is present in human serum, is associated with VLDL, HDL, and chylomicrons, and circulates at very low concentrations compared with other apolipoproteins.Clin. Chem. 2005; 51: 351-359Crossref PubMed Scopus (182) Google Scholar). In apoA-V transfected cells, however, the protein was readily detected, with roughly similar amounts present in cell lysate and medium. By contrast, nearly all of the apoA-I and most of the apoE and apoB-100 detected was in the medium. The similar apolipoprotein distribution seen in control vector and apoA-V transfected cells indicates that overexpression of apoA-V does not affect the secretion efficiency of these apolipoproteins. At the same time, however, the difference in apolipoprotein distribution between apoA-V and the other apolipoproteins suggests that its secretion is impaired. In murine models, apoA-V exerts a strong influence on plasma TG levels. Thus, it is conceivable that, although apoB-100 secretion from Hep3B cells is unaffected by apoA-V transfection, TG-rich lipoprotein particle density distribution may be altered. To examine this conditioned medium from Hep3B cell was subjected to cumulative rate flotation centrifugation followed by immunoprecipitation with an antibody directed apoB-100 (Fig. In the of Hep3B cells, apoB is in the density with a in similar apoB distribution was in cell transfected with empty vector or apoA-V that apoA-V does not influence the lipidation of apoB secreted from these Immunoprecipitation of apoA-V (Fig. indicated that apoA-V is associated with in the density amount of apoA-V is found in and In Hep3B cell apoA-V was present in the density therefore, we apoA-V and apoB are on the same lipoprotein apoA-V and apoB from the and immunoprecipitation with anti-apoB apoB and apoA-V (Fig. The that apoA-V with apoB and is consistent with the observation that apoA-V is found on VLDL in plasma (4.O'Brien P.J. Alborn W.E. Sloan J.H. Ulmer M. Boodhoo A. Knierman M.D. Schultze A.E. Konrad R.J. The novel apolipoprotein A5 is present in human serum, is associated with VLDL, HDL, and chylomicrons, and circulates at very low concentrations compared with other apolipoproteins.Clin. Chem. 2005; 51: 351-359Crossref PubMed Scopus (182) Google Scholar). Hep3B cells expressing were lysed and the lysate was with anti-FLAG and anti-apoB antibody apoA-V but not apoB from the lysate (Fig. apoB antibody apoB but not suggesting that intracellularly, apoB and apoA-V are not Hep3B cells have low levels of intracellular apoB (Fig. McA-RH7777 cells expressing levels of human apoB-100 were transfected with an apoA-V-GFP vector and used to apoA-V and apoB-100 Confocal (Fig. of fixed cells with anti-apoB followed by an Alexa Fluor 594-labeled antibody confirmed the of this apolipoprotein to the (18.Tran K. Thorne-Tjomsland G. DeLong C.J. Cui Z. Shan J. Burton L. Jamieson J.C. Yao Z. Intracellular assembly of very low density lipoproteins containing apolipoprotein B100 in rat hepatoma McA-RH7777 cells.J. Biol. Chem. 2002; 277: 31187-31200Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar). the other apoA-V-GFP a very distribution distinct cytosolic structures that a with of apoB and apoA-V was (Fig. consistent with the immunoprecipitation studies with cell Based on the distribution of apoA-V and its to cytosolic we hypothesized that apoA-V may be associated with intracellular lipid droplets. To lipid were with Nile Red and apoA-V was by GFP fluorescence (Fig. seen in apoA-V to the of lipid that stain with Nile Red (Fig. The shown in the of apoA-V and intracellular lipid droplets. studies with apoA-V and FLAG-tagged apoA-V indicated that the was not to the GFP Cells transfected with revealed that, this protein not to lipid but was associated with the These data suggest of apoA-V for lipid droplets. Evidence from mice (2.Pennacchio L.A. Olivier M. Hubacek J.A. Cohen J.C. Cox D.R. Fruchart J.C. Krauss R.M. Rubin E.M. An apolipoprotein influencing triglycerides in humans and mice revealed by comparative sequencing.Science. 2001; 294: 169-173Crossref PubMed Scopus (801) Google Scholar) and human studies (4.O'Brien P.J. Alborn W.E. Sloan J.H. Ulmer M. Boodhoo A. Knierman M.D. Schultze A.E. Konrad R.J. The novel apolipoprotein A5 is present in human serum, is associated with VLDL, HDL, and chylomicrons, and circulates at very low concentrations compared with other apolipoproteins.Clin. Chem. 2005; 51: 351-359Crossref PubMed Scopus (182) Google Scholar, C. Verges B. Charriere S. Pruneta V. Merlin M. Billon S. Perrot L. Drai J. Sassolas A. Pennacchio L.A. et al.Apoa5 Q139X truncation predisposes to late-onset hyperchylomicronemia due to lipoprotein lipase impairment.J. Clin. Invest. 2005; 115: 2862-2869Crossref PubMed Scopus (140) Google Scholar) has revealed that apoA-V plays a role in TG levels. The mechanism whereby apoA-V TG metabolism is not One of suggested that apoA-V by lipoprotein the efficiency of and VLDL-TG (12.Schaap F.G. Rensen P.C. Voshol P.J. Vrins C. van der Vliet H.N. Chamuleau R.A. Havekes L.M. Groen A.K. van Dijk K.W. ApoAV reduces plasma triglycerides by inhibiting very low density lipoprotein-triglyceride (VLDL-TG) production and stimulating lipoprotein lipase-mediated VLDL-TG hydrolysis.J. Biol. Chem. 2004; 279: 27941-27947Abstract Full Text Full Text PDF PubMed Scopus (259) Google Scholar, 13.Merkel M. Loeffler B. Kluger M. Fabig N. Geppert G. Pennacchio L.A. Laatsch A. Heeren J. Apolipoprotein AV accelerates plasma hydrolysis of triglyceride-rich lipoproteins by interaction with proteoglycan-bound lipoprotein lipase.J. Biol. Chem. 2005; 280: 21553-21560Abstract Full Text Full Text PDF PubMed Scopus (249) Google Scholar, A. Beckstead J.A. S. G. Ryan R.O. Apolipoprotein for plasma lipoprotein Biol. Chem. 2005; 280: Full Text Full Text PDF PubMed Scopus Google Scholar). Given the low concentration of apoA-V in plasma, it is that only a of VLDL will an apoA-V the regulation of a regulator of plasma Thromb. Vasc. Biol. 2005; 25: PubMed Scopus Google Scholar). the that on plasma TG may be intracellular rather than in the plasma In this we that apoA-V secretion from cultured Hep3B cells is but that its overexpression has no effect on apoB secretion or lipidation. observation is consistent with that of Weinberg et al. (14.Weinberg R.B. Cook V.R. Beckstead J.A. Martin D.D. Gallagher J.W. Shelness G.S. Ryan R.O. Structure and interfacial properties of human apolipoprotein A-V.J. Biol. Chem. 2003; 278: 34438-34444Abstract Full Text Full Text PDF PubMed Scopus (136) Google Scholar), who that apoA-V expressed in COS-1 cells a low secretion efficiency compared with albumin and a truncated of human apoB. secretion of apoA-V may the low concentration of this protein in humans (4.O'Brien P.J. Alborn W.E. Sloan J.H. Ulmer M. Boodhoo A. Knierman M.D. Schultze A.E. Konrad R.J. The novel apolipoprotein A5 is present in human serum, is associated with VLDL, HDL, and chylomicrons, and circulates at very low concentrations compared with other apolipoproteins.Clin. Chem. 2005; 51: 351-359Crossref PubMed Scopus (182) Google Scholar). The density distribution of apoB-containing lipoproteins secreted by Hep3B cells transfected with apoA-V was from that of cells transfected with an empty vector, that apoA-V overexpression does not influence apoB-containing lipoprotein particle secretion or lipidation. is from reported by Schaap et al. (12.Schaap F.G. Rensen P.C. Voshol P.J. Vrins C. van der Vliet H.N. Chamuleau R.A. Havekes L.M. Groen A.K. van Dijk K.W. ApoAV reduces plasma triglycerides by inhibiting very low density lipoprotein-triglyceride (VLDL-TG) production and stimulating lipoprotein lipase-mediated VLDL-TG hydrolysis.J. Biol. Chem. 2004; 279: 27941-27947Abstract Full Text Full Text PDF PubMed Scopus (259) Google Scholar), who found that VLDL but not particle number, was by adenovirus-mediated apoA-V overexpression in it is that apoA-V with VLDL in plasma, we used confocal microscopy to determine an association between apoA-V and apoB microscopy and immunoprecipitation of cell lysates that apoB and apoA-V are in and distinct cell suggesting that the apoA-V association with VLDL is a postsecretory event. The with observation that most of the secreted apoA-V is found in HDL, suggests that apoA-V may be from the cell on and VLDL secretion. apoB to the and the secretory apoA-V-GFP in a that not colocalize with apoB. the confocal microscopy data suggest that an intracellular of apoA-V that the secretory the that it is synthesized with a secretory signal In this apoA-V is similar to the described which is retained in cells it a signal M. R. P. A. J. M. X. P. et a novel is an by in human Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). to lipid within In a similar apoA-V is retained in and to cytosolic lipid droplets. Given that TG is the lipid of intracellular lipid it is conceivable that on plasma TG levels are its with this cellular lipid The Dr. and for was by of with

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