John R. Wetterau

Microsomal triglyceride transfer protein

Nucleic acid sequences, particularly DNA sequences, coding for all or part of the high molecular weight subunit of microsomal triglyceride transfer protein, expression vectors containing the DNA sequences, host cells containing the expression vectors, and methods utilizing these materials. The invention also concerns polypeptide molecules comprising all or part of the high molecular weight subunit of microsomal triglyceride transfer protein, and methods for producing these polypeptide molecules. The invention additionally concerns novel methods for preventing, stabilizing or causing regression of atherosclerosis and therapeutic agents having such activity. The invention concerns further novel methods for lowering serum liquid levels and therapeutic agents having such activity.Nucleic acid sequences, particularly DNA sequences, coding for all or part of the high molecular weight subunit of microsomal triglyceride transfer protein, expression vectors containing the DNA sequences, host cells containing the expression vectors, and methods utilizing these materials. The invention also concerns polypeptide molecules comprising all or part of the high molecular weight subunit of microsomal triglyceride transfer protein, and methods for producing these polypeptide molecules. The invention additionally concerns novel methods for preventing, stabilizing or causing regression of atherosclerosis and therapeutic agents having such activity. The invention concerns further novel methods for lowering serum liquid levels and therapeutic agents having such activity.

Microsomal triglyceride transfer protein: a protein complex required for the assembly of lipoprotein particles

The mechanism of assembly of lipoprotein particles in the lumen of the endoplasmic reticulum is an important but poorly understood biological problem. A knowledge of this process is of great practical importance because possession of elevated levels of lipoproteins is one of the major risk factors for the development of atherosclerosis. This review describes a major advance in the delineation of the mechanisms involved in the assembly and secretion of apolipoprotein-B-containing lipoproteins: the demonstration of a requirement for microsomal triglyceride transfer protein.

The Amino Terminus of Apolipoprotein B Is Necessary but Not Sufficient for Microsomal Triglyceride Transfer Protein Responsiveness

Human apolipoprotein (apo) B mediates the formation of neutral lipid-containing lipoproteins in the liver and intestine. The association of apoB with lipid is thought to be promoted by the microsomal triglyceride transfer protein complex. We have reconstituted lipoprotein assembly in an insect cell line that normally does not support this process. Expression of human microsomal triglyceride transfer protein (MTP) and apolipoprotein B48 (apoB48) together enabled Sf-21 insect cells to secrete 60-fold more lipoprotein-associated triacylglycerol than control cells. This dramatic effect demonstrates that effective partitioning of triacylglycerol into the secretory pathway requires an endoplasmic reticulum-associated neutral lipid transporter (provided by MTP) and an apolipoprotein to shuttle the lipid through the pathway. Expression of the human apoB48 gene in insect cells resulted in secretion of the protein product. Including both MTP subunits with apoB48 and oleic acid specifically increased apoB48 secretion 8-fold over individual subunits alone. To assess whether specific regions of apoB are necessary for MTP responsiveness, nine apoB segments were expressed. These included NH-terminal segments as well as internal and COOH-terminal regions of apoB fused with a heterologous signal sequence. ApoB segments containing the NH-terminal 17% of the protein were secreted and responded to MTP activity; however, a segment containing only the NH-terminal 17% of the protein was not significantly responsive to MTP. Segments lacking the NH terminus were not MTP-responsive, and five of six of these proteins were trapped intracellularly but, in certain cases, could be rescued by fusion to apoB17. These results suggest that the NH terminus of apoB is necessary but not sufficient for MTP responsiveness.

A Novel Abetalipoproteinemia Genotype IDENTIFICATION OF A MISSENSE MUTATION IN THE 97-kDa SUBUNIT OF THE MICROSOMAL TRIGLYCERIDE TRANSFER PROTEIN THAT PREVENTS COMPLEX FORMATION WITH PROTEIN DISULFIDE ISOMERASE

The microsomal triglyceride transfer protein (MTP) is a heterodimer composed of the ubiquitous multifunctional protein, protein disulfide isomerase, and a unique 97-kDa subunit. Mutations that lead to the absence of a functional 97-kDa subunit cause abetalipoproteinemia, an autosomal recessive disease characterized by a defect in the assembly and secretion of apolipoprotein B (apoB) containing lipoproteins. Previous studies of abetalipoproteinemic patient, C.L., showed that the 97-kDa subunit was undetectable. In this report, [35S]methionine labeling showed that this tissue was capable of synthesizing the 97-kDa MTP subunit. Electrophoretic analysis showed two bands, one with a molecular mass of the wild type 97-kDa subunit and the other with a slightly lower molecular weight. Sequence analysis of cDNAs from additional intestinal biopsies showed this patient to be a compound heterozygote. One allele contained a perfect in-frame deletion of exon 10, explaining the lower molecular weight band. cDNAs of the second allele were found to contain 3 missense mutations: His297 → Gln, Asp384 → Ala, and Arg540 → His. Transient expression of each mutant showed that only the Arg540 → His mutant was non-functional based upon its inability to reconstitute apoB secretion in a cell culture system. The other amino acid changes are silent polymorphisms. High level coexpression in a baculovirus system of the wild type 97-kDa subunit or the Arg540 → His mutant along with human protein disulfide isomerase showed that the wild type was capable of forming an active MTP complex while the mutant was not. Biochemical analysis of lysates from these cells showed that the Arg to His conversion interrupted the interaction between the 97-kDa subunit and protein disulfide isomerase. Replacement of Arg540 with a lysine residue maintained the ability of the 97-kDa subunit to complex with protein disulfide isomerase and form the active MTP holoprotein. These results indicate that a positively charged amino acid at position 540 in the 97-kDa subunit is critical for the productive association with protein disulfide isomerase. Of the 13 mutant MTP 97-kDa subunit alleles described to date, this is the first encoding a missense mutation.

Microsomal triglyceride transfer protein inhibitors: discovery and synthesis of alkyl phosphonates as potent MTP inhibitors and cholesterol lowering agents

A series of newly synthesized phosphonate esters were evaluated for their effects on microsomal triglyceride transfer protein activity (MTP). The most potent compounds were evaluated for their ability to inhibit lipoprotein secretion in HepG2 cells and to affect VLDL secretion in rats. These inhibitors were also found to lower serum cholesterol levels in a hamster model upon oral dosing.

Crystallization of microsomal triglyceride transfer protein from bovine liver

The microsomal triglyceride transfer protein (MTP) is a heterodimeric lipid transfer protein required for the assembly of plasma very low density lipoproteins in the liver and chylomicrons in the intestine. Bovine MTP was purified by a modification of a previously published procedure and crystals of MTP were grown reproducibly with polyethylene glycol as a precipitant at pH 7.0. MTP crystals, which diffract to Bragg spacings of better than 3.2 A, have the symmetry of space group P2(1)2(1)2(1) with refined lattice constants of a = 88.7, b = 100.9 and c = 201.1 A, with one heterodimer per asymmetric unit.