A deficiency in microsomal triglyceride transfer protein (MTP) causes the human lipoprotein deficiency syndrome abetalipoproteinemia. However, the role of MTP in the assembly and secretion of VLDL in the liver is not precisely understood. It is not clear, for instance, whether MTP is required to move the bulk of triglycerides into the lumen of the endoplasmic reticulum (ER) during the assembly of VLDL particles. To define MTP's role in hepatic lipoprotein assembly, we recently knocked out the mouse MTP gene (Mttp). Unfortunately, achieving our objective was thwarted by a lethal embryonic phenotype. In this study, we produced mice harboring a "floxed" Mttp allele and then used Cre-mediated recombination to generate liver-specific Mttp knockout mice. Inactivating the Mttp gene in the liver caused a striking reduction in VLDL triglycerides and large reductions in both VLDL/LDL and HDL cholesterol levels. The Mttp inactivation lowered apo B-100 levels in the plasma by >95% but reduced plasma apo B-48 levels by only ∼20%. Histologic studies in liver-specific knockout mice revealed moderate hepatic steatosis. Ultrastructural studies of wild-type mouse livers revealed numerous VLDL-sized lipid-staining particles within membrane-bound compartments of the secretory pathway (ER and Golgi apparatus) and few cytosolic lipid droplets. In contrast, VLDL-sized lipid-staining particles were not observed in MTPdeficient hepatocytes, either in the ER or in the Golgi apparatus, and there were numerous cytosolic fat droplets. We conclude that MTP is essential for transferring the bulk of triglycerides into the lumen of the ER for VLDL assembly and is required for the secretion of apo B-100 from the liver.J. Clin. Invest. 103:1287-1298(1999 to play an important role in this first "apo B lipidation" step (1,21,25). The existence of apo B in the rough ER of hepatocytes has been documented by immunoelectron microscopy (16). However, the lipidated apo B particles cannot be seen in the rough ER by routine electron microscopy, even when the thin sections are stained for lipids, because those particles are too small (< 150 Å in diameter) and lipid poor to be resolved by this technique. In a second step, the lipidated apo B molecule is thought to acquire the bulk of its core lipids by fusing with a large, VLDL-sized, apo B-free triglyceride particle (a "second-step" particle) (17,23). The existence of the secondstep triglyceride particles within a special compartment of the smooth ER has been supported by 2 different electron microscopic studies (16,23). Biochemical studies of VLDL assembly have also supported the concept that the bulk of neutral lipids are added to apo B in a second step after its translation is complete (1, 24).The role, if any, of MTP in the formation of second-step lipid particles is unclear. In fact, this issue has recently been highlighted as one of the fundamental problems in understanding MTP and lipoprotein assembly (15,26). In recent years, several groups have tried to address this issue by performing metabolic la...

IntroductionThe recognition of hypercholesterolemia as a risk factor for atherosclerosis, and the identification of different classes of cholesterol-carrying lipoproteins, have spurred efforts to identify the most atherogenic lipoproteins. Over the past 40 years, clinical, epidemiological, genetic, and experimental animal studies have demonstrated that the apo-B-containing lipoproteins promote atherogenesis (1). However, the apo-B-containing lipoproteins are heterogeneous both in size and composition, and there has been controversy regarding which class of these lipoproteins is most atherogenic (2, 3). Much of this discussion has focused on whether particle size affects the intrinsic capacity of lipoproteins to promote atherogenesis. LDLs, which are at the small end of the size spectrum, are clearly atherogenic. LDLs are the principal cholesterol-carrying lipoproteins in the plasma of most human patients with coronary disease (1), and elevated LDL levels in familial hypercholesterolemia (FH) cause severe atherosclerosis (4). On the other hand, ample evidence indicates that larger particles (i.e., chylomicron and VLDL remnants) can be atherogenic. Cholesterolrich remnant lipoproteins accumulate in type III hyperlipoproteinemia, and affected individuals are at risk for atherosclerosis (5). In addition, population studies have uncovered links between elevated plasma levels of cholesterol-rich remnant particles and increased risk of coronary heart disease (6).Studies in humans support the notion that both remnants and small LDL are atherogenic. However, these studies have not provided a clear answer to the question of whether different-sized lipoproteins differ in their intrinsic capacities to promote atherosclerosis. Thus, at a given total plasma cholesterol level, it is still not clear whether it matters -from the standpoint of atherogenesis -whether the cholesterol is carried in LDLs or in the larger remnant lipoproteins. This is a difficult issue to address with human studies. Many coronary disease patients have high levels of both remnants and LDL as well as other risk factors (e.g., diabetes mellitus, obesity, low HDL levels), making it difficult to sort out the importance of lipoprotein size. Studies of human genetic diseases have also failed to settle the issue of the intrinsic atherogenicities of large and small lipoproteins. Even if one assembled large groups of subjects with type III hyperlipoproteinemia and FH that were perfectly matched for total plasma cholesterol levels and other risk factors, defining the prevalence of athero-

We recently reported that an 8-kilobase (kb) region, spanning from ؊54 to ؊62 kb 5 of the human apolipoprotein B (apoB) gene, contains intestine-specific regulatory elements that control apoB expression in the intestines of transgenic mice. In this study, we further localized the apoB intestinal control region to a 3-kb segment (؊54 to ؊57 kb). DNaseI hypersensitivity studies uncovered a prominent DNaseI hypersensitivity site, located within a 315-base pair ( In recent years, valuable information regarding the molecular mechanisms regulating tissue-specific transcriptional control of mammalian genes has emerged (for a recent review see Ref.

No abstract