Apolipoprotein B (apoB) is responsible for the packaging of dietary and endogenous lipids into lipoprotein particles. These particles are distributed to peripheral tissues, such as muscle and adipose, where the lipids are either utilized for energy or stored. However, intravascular metabolism of apoB-containing lipoproteins, particular those originating from liver, give rise to low density lipoproteins (LDL), which in elevated concentrations in the blood cause atherosclerosis and heart disease. Furthermore, imbalances among hepatic and intestinal lipid biosynthesis, storage, utilization and secretion are important in the pathobiology of obesity, type 2 diabetes, and nonalcoholic fatty liver disease.
Our laboratory is studying the process by which apoB, in combination with the microsomal triglyceride transfer protein (MTP) and other cofactors, controls lipid transport by the liver and intestine. Our most recent studies have focused on the unexpected evolutionary origins of lipid transport proteins. It is now clear that MTP is the ancestral member of this gene family and exists in divergent vertebrate and invertebrate species, whose last common ancestor diverged over 550 million years ago. During its long evolutionary history, MTP has acquired distinct functions enabling it to participate in a disparate array of lipid mobilization and transport pathways, ranging from primitive lipoprotein assembly in nematodes and arthropods, to bulk lipid transport and antigenic lipid presentation in humans. Our phylogenic dissection of MTP and apoB function coupled with ongoing structural, biochemical, and biophysical analyses, are providing new insights into mechanisms of lipid mobilization and secretion and strategies to beneficially control disturbances in lipid metabolism associated with chronic disease.
As an outgrowth of our studies on the structure and function of apoB, we are also exploring the use of apoB’s lipid binding domains to achieve the packaging of lipophilic drugs. By fusing apoB to single chain antibodies unique to transformed cells, we hope to achieve the selective delivery of drug-containing complexes to cancer cells without affecting healthy cells and tissues.
Figure Legend: Lipid Mobilization and Secretion by Lipoprotein Producing Cells. Neutral lipids such as triglycerides and cholesterol esters are synthesized by enzymes associated with the endoplasmic reticulum (ER) membrane (1). These lipids are either stored in the cytosol or consumed for energy production (Utilization). Lipid secretion requires the mobilization of cytosolic lipid, a step that involves cytosolic and membrane-associated factors that are not well defined (2). Microsomal triglyceride transfer protein (MTP), which resides in the lumen of the ER, possesses a neutral lipid transfer activity that is required for the generation of lumenal lipid droplets. MTP also functions directly on apolipoprotein B (apoB) to form precursor lipoprotein particles (5). These precursor particles fuse with lipid droplets to form mature intestinal chylomicrons of hepatic VLDL (6), which are then secreted via the classical secretory pathway (7).
Brown JM, Chung S, Das A, Shelness GS, Rudel LL, Yu L. CGI-58 facilitates the mobilization of cytoplasmic triglyceride for lipoprotein secretion in hepatoma cells. J Lipid Res. 2007 Oct;48(10):2295-305.
Mohler PJ, Zhu MY, Blade AM, Ham AJ, Shelness GS, Swift LL. Identification of a novel isoform of microsomal triglyceride transfer protein. J Biol Chem. 2007 Sep 14;282(37):26981-8.
Temel RE, Hou L, Rudel LL, Shelness GS. ACAT2 stimulates cholesteryl ester secretion in apoB-containing lipoproteins. J Lipid Res. 2007 Jul;48(7):1618-27.
Krugner-Higby L, Shelness GS, Holler A. Heritable, diet-induced hyperlipidemia in California mice (Peromyscus californicus) is due to increased hepatic secretion of very low density lipoprotein triacylglycerol. Comp Med. 2006 Dec;56(6):468-75.
Rava P, Ojakian GK, Shelness GS, Hussain MM. Phospholipid transfer activity of microsomal triglyceride transfer protein is sufficient for the assembly and secretion of apoB-lipoproteins. J Biol Chem. 2006 Apr 21;281(16):11019-27.
Ledford AS, Weinberg RB, Cook VR, Hantgan RR, Shelness GS. Self-association and lipid binding properties of the lipoprotein initiating domain of apolipoprotein B. J Biol Chem. 2006 Mar 31;281(13):8871-6.
Lu S, Yao Y, Cheng X, Mitchell S, Leng S, Meng S, Gallagher JW, Shelness GS, Morris GS, Mahan J, Frase S, Mansbach CM, Weinberg RB, Black DD. Overexpression of Apolipoprotein A-IV Enhances Lipid Secretion in IPEC-1 Cells by Increasing Chylomicron Size. J Biol Chem. 2006 Feb 10;281(6):3473-83.
Shelness GS, Rudel LL. A role for the pregnane X receptor in high-density lipoprotein metabolism. Arterioscler Thromb Vasc Biol. 2005 Oct;25(10):2016-7.
Shelness GS, Ledford AS. Evolution and mechanism of apolipoprotein B-containing lipoprotein assembly. Curr Opin Lipidol. 2005 Jun;16(3):325-32.
Shoulders CC, Shelness GS. Current biology of MTP: implications for selective inhibition. Curr Top Med Chem. 2005(3):283-300. Review.
Sellers JA, Hou L, Schoenberg DR, Batistuzzo de Medeiros SR, Wahli W, Shelness GS. Microsomal triglyceride transfer protein promotes the secretion of Xenopus laevis vitellogenin A1. J Biol Chem. 2005 Apr 8;280(14):13902-5.
Snipes JA, Kis B, Shelness GS, Hewett JA, Busija DW. Cloning and characterization of cyclooxygenase-1b (putative cyclooxygenase-3) in rat. J Pharmacol Exp Ther. 2005 May;313(2):668-76.
Shelness GS, Dawson PA. In memoriam: David L. Williams 1946-2004. J Lipid Res. 2004 Dec;45(12):2388-9.
Gallagher JW, Weinberg RB, Shelness GS. Apolipoprotein AIV tagged with the C-terminal ER retention signal KDEL perturbs the intracellular trafficking and secretion of apolipoprotein B. J Lipid Res. 2004 Oct;45(10):1826-34.
Bhat S, Zabalawi M, Willingham MC, Shelness GS, Thomas MJ, Sorci-Thomas MG. Quality control in the apoA-I secretory pathway: deletion of apoA-I helix 6 leads to the formation of cytosolic phospholipid inclusions. J Lipid Res. 2004 Jul;45(7):1207-20.
Shelness GS, Hou L, Ledford AS, Parks JS, Weinberg RB. Identification of the lipoprotein initiating domain of apolipoprotein B. J Biol Chem. 2003 Nov 7;278(45):44702-7.
Weinberg RB, Cook VR, Beckstead JA, Martin DD, Gallagher JW, Shelness GS, Ryan RO. Structure and interfacial properties of human apolipoprotein A-V. J Biol Chem. 2003 Sep 5;278(36):34438-44.
Sellers JA, Hou L, Athar H, Hussain MM, Shelness GS. A Drosophila microsomal triglyceride transfer protein homolog promotes the assembly and secretion of human apolipoprotein B. Implications for human and insect lipid transport and metabolism. J Biol Chem. 2003 May 30;278(22):20367-73.
Chisholm JW, Burleson ER, Shelness GS, Parks JS. ApoA-I secretion from HepG2 cells: evidence for the secretion of both lipid-poor apoA-I and intracellularly assembled nascent HDL. J Lipid Res. 2002 Jan;43(1):36-44.
Sellers JA, Shelness GS. Lipoprotein assembly capacity of the mammary tumor-derived cell line C127 is due to the expression of functional microsomal triglyceride transfer protein. J Lipid Res. 2001 Nov;42(11):1897-904.
Shelness GS, Sellers JA. Very-low-density lipoprotein assembly and secretion. Curr Opin Lipidol. 2001 Apr;12(2):151-7. Review.
DeLozier JA, Parks JS, Shelness GS. Vesicle-binding properties of wild-type and cysteine mutant forms of alpha(1) domain of apolipoprotein B. J Lipid Res. 2001 Mar;42(3):399-406.
Rudel LL, Shelness GS. Cholesterol esters and atherosclerosis-a game of ACAT and mouse. Nat Med. 2000 Dec;6(12):1313-4. No abstract available.
Joyce CW, Shelness GS, Davis MA, Lee RG, Skinner K, Anderson RA, Rudel LL. ACAT1 and ACAT2 membrane topology segregates a serine residue essential for activity to opposite sides of the endoplasmic reticulum membrane. Mol Biol Cell. 2000 Nov;11(11):3675-87.
Reagan JW Jr, Hubbert ML, Shelness GS. Posttranslational regulation of acid sphingomyelinase in niemann-pick type C1 fibroblasts and free cholesterol-enriched chinese hamster ovary cells. J Biol Chem. 2000 Dec 1;275(48):38104-10.
Weinberg RB, Cook VR, DeLozier JA, Shelness GS. Dynamic interfacial properties of human apolipoproteins A-IV and B-17 at the air/water and oil/water interface. J Lipid Res. 2000 Sep;41(9):1419-27.
Davidson NO, Shelness GS. APOLIPOPROTEIN B: mRNA editing, lipoprotein assembly, and presecretory degradation. Annu Rev Nutr. 2000;20:169-93. Review.
Hussain MM, Obunike JC, Shaheen A, Hussain MJ, Shelness GS, Goldberg IJ. High affinity binding between lipoprotein lipase and lipoproteins involves multiple ionic and hydrophobic interactions, does not require enzyme activity, and is modulated by glycosaminoglycans. J Biol Chem. 2000 Sep 22;275(38):29324-30.