Biotinylation of lipoprotein lipase and hepatic triglyceride lipase: application in the assessment of cell binding sites

Analysis and quantitation of biotinylated apoB-containing lipoproteins with streptavidin-Cy3

Non denaturing gradient gel electrophoresis (GGE) is commonly used to analyze the size distribution of lipoprotein particles. Its relatively low sensitivity and linear dynamic range limit use of GGE to quantify protein content of lipoproteins. We demonstrate a new high sensitivity method for analysis and quantitation of biotinylated apolipoprotein B (apoB)-containing lipoproteins using a fluorescent streptavidin-Cy3 conjugate and non covalent preelectrophoretic binding. Forty-four lipoprotein subfractions spanning the VLDL and LDL particle spectrum subfractions (11 each from four human subjects) were prepared by density gradient ultracentrifugation. An aliquot of each sample was biotinylated and GGE was performed. Gels also were stained for lipid with Oil Red O (32 samples) and for protein with Coomassie Brilliant Blue (30 samples). There was a significant relationship between the Cy3 fluorescent label area under the curve and the mass of apoB (P < 0.02-0.004) and total cholesterol (P < 0.03-0.004). Particle diameters of each absorbence/fluorescent peak were comparable between Oil-Red O and streptavidin-Cy3 treated biotinylated lipoproteins (+/-3.54 A, P = 0.3). Biotinylation and prestaining of lipoprotein particle with streptavidin-Cy3 provides a new fluorescence-based method for detection and quantitative analysis of lipoprotein subspecies by gradient gel electrophoresis.

Calcium and lipoprotein lipase synergistically enhance the binding and uptake of native and oxidized LDL in mouse peritoneal macrophages

The influence of Ca(2+) and Mg(2+), together with lipoprotein lipase (LPL), on the binding and uptake of Eu(3+)-labeled native and oxidized low density lipoprotein (LDL) to mouse peritoneal macrophages (MPM), and on the deposition of esterified cholesterol in these macrophages, were studied. We found that both LPL and Ca(2+) (but not Mg(2+)) increased the binding and uptake of native and mildly or moderately oxidized LDL, and the subsequent deposition of cholesterol esters in MPM. When added together, LPL and Ca(2+) synergistically increased the binding and uptake of native and oxidized LDL, and the deposition of esterified cholesterol derived from native and mildly or moderately oxidized LDL, in MPM. Since both calcium and LPL are found in the atherosclerotic lesions, our results suggest that Ca(2+) and LPL may synergistically promote foam cell formation and atherogenesis. Furthermore, future research in the metabolism of lipoproteins should take into account the calcium levels in the experimental conditions.

Endothelial cells synthesize and process apolipoprotein B

We reported previously that a 116-kDa lipoprotein lipase (LPL)-binding protein from endothelial cells has sequence homology to the amino-terminal region of apolipoprotein (apo) B. We now tested whether endothelial cells synthesize apoB mRNA and protein. Primers were designed to the human apoB cDNA sequence and reverse transcription polymerase chain reaction was performed using total RNA isolated from bovine and human endothelial cells. With primers to the 5' region of the apoB mRNA (amino-terminal region of apoB protein) expected size PCR products were generated from both bovine and human endothelial cells as well as from mouse liver RNA, which was used as a control. Primers designed to the 3' region of apoB mRNA generated PCR products from human endothelial cells and HepG2 cells but not from bovine or mouse cells. These data suggest that endothelial cells contain full-length apoB mRNA and that the 5' or the amino-terminal region of apoB is highly conserved from mouse to human. This was confirmed by direct sequencing of the mouse and bovine PCR products. To test whether apoB protein was produced, bovine endothelial cell proteins were metabolically labeled with [35S]methionine/cysteine or [3H]leucine and immunoprecipitated with anti-human apoB antibodies. Using extracts from cells labeled for 1 h, monoclonal antibody 47, directed to the low density lipoprotein receptor binding region of apoB, precipitated a protein of approximate molecular mass 550,000, the size of full-length apoB. Immunoprecipitation of the 550-kDa protein was abolished in the presence of added unlabeled low density lipoprotein. From cells labeled for 16 h, a 116-kDa protein was immunoprecipitated by polyclonal anti-apoB antibodies. This protein was partly released from cells by heparin treatment. Pulse-chase analysis showed that the 116-kDa fragment appeared at the same time as the full-length apoB began disappearing. The immunoprecipitated 116-kDa fragment also bound labeled LPL on ligand blot, further suggesting that it is an amino-terminal fragment of apoB. Incubation of endothelial cells with oleic acid (0.25 and 0.5 mM) did not significantly alter the production of either the full-length apoB or the 116-kDa fragment. These data show that endothelial cells synthesize apoB. The full-length apoB appears to be cleaved to form a 116-kDa fragment that can function as a LPL-binding protein.

Lipoprotein lipase association with lipoproteins involves protein-protein interaction with apolipoprotein B

Lipoprotein lipase (LPL) hydrolyzes chylomicron and very low density lipoprotein (VLDL) triglycerides and potentiates the cellular uptake of lipoproteins. These LPL-lipoprotein associations could involve only protein-lipid interaction, or they could be modulated by apolipoproteins (apo). ApoB is the major protein component of chylomicrons, VLDL, and low density lipoprotein (LDL). ApoB100, a large glycoprotein with a molecular mass of 550 kDa, is composed of several functional domains. A carboxyl-terminal region of the protein is the ligand for the LDL receptor. There are several hydrophobic domains that are believed to be important in lipid binding. The relatively hydrophilic amino-terminal region of apoB, however, has no known function. Using solid phase assays we quantified LPL-lipoprotein complex formation. On a molar basis, severalfold greater amounts of LPL bound to LDL and VLDL than to high density lipoprotein at all the concentrations of LPL tested (0.9-55 nM). To assess the roles of LDL protein versus lipid, we performed competition and ligand blotting experiments. LDL and an amino-terminal fragment of apoB competed better for 125I-LPL binding to LDL than did lipid emulsion particles. Delipidation of LDL-coated plates did not alter LPL binding. On ligand blots, LPL bound to amino-terminal fragments of apoB generated by thrombin digestion but not to apoA1, apoE, or carboxyl-terminal fragments of apoB. Further evidence for LPL interaction with the amino-terminal region of apoB was obtained using anti-apoB monoclonal antibodies. Antibodies directed against the amino-terminal regions of apoB blocked LPL interaction with LDL, whereas those against the carboxyl-terminal region of apoB did not inhibit LPL interaction with LDL. Thus, we conclude that a specific interaction between LPL and the amino-terminal region of apoB may facilitate LPL association with circulating lipoproteins.