title: Molecular cloning, expression, and hormonal regulation of the chicken microsomal triglyceride transfer protein.
abstract: During an egg-laying cycle, oviparous animals transfer massive amounts of triglycerides, the major lipid component of very low density lipoprotein (VLDL), from the liver to the developing oocytes. A major stimulus for this process is the rise in estrogen associated with the onset of an egg-laying cycle. In mammals, the microsomal triglyceride transfer protein (MTP) is required for VLDL assembly and secretion. To enable studies to determine if MTP plays a role in basal and estrogen-stimulated VLDL assembly and secretion in an oviparous vertebrate, we have cloned and sequenced the chicken MTP cDNA. This cDNA encodes a protein of 893 amino acids with an N-terminal signal sequence. The primary sequence of chicken MTP is, on average, 65% identical to that of mammalian homologs, and 23% identical to the Drosophila melanogaster protein. We have obtained a clone of chicken embryo fibroblast cells that stably express the avian MTP cDNA and show that these cells display MTP activity as measured by the transfer of a fluorescently labeled neutral lipid. As in mammals, chicken MTP is localized to the endoplasmic reticulum as revealed by indirect immunofluorescence and by the fact that its N-linked oligosaccharide moiety remains sensitive to endoglycosidase H. Endogenous, enzymatically active MTP is also expressed in an estrogen receptor-expressing chicken hepatoma cell line that secretes apolipoprotein B-containing lipoproteins. In this cell line and in vivo, the expression and activity of MTP are not influenced by estrogen. Therefore, up-regulation of MTP in the liver is not required for the increased VLDL assembly during egg production in the chicken. This indicates that MTP is not rate-limiting, even for the massive estrogen-induced secretion of VLDL accompanying an egg-laying cycle.
authors.affiliation: Department of Medical Biochemistry, Medical University Vienna, Max F. Perutz Laboratories, A-1030 Vienna, Austria. n-erwin.ivessa@meduniwien.ac.at
authors.forename: N Erwin
authors.initials: NE
authors.lastname: Ivessa
authors.forename: Edward
authors.initials: E
authors.lastname: Rehberg
authors.forename: Bernadette
authors.initials: B
authors.lastname: Kienzle
authors.forename: Fridolin
authors.initials: F
authors.lastname: Seif
authors.forename: Robert
authors.initials: R
authors.lastname: Hermann
authors.forename: Marcela
authors.initials: M
authors.lastname: Hermann
authors.forename: Wolfgang J
authors.initials: WJ
authors.lastname: Schneider
authors.forename: David A
authors.initials: DA
authors.lastname: Gordon
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001055:Apolipoproteins B / Q000378:metabolism; D001692:Biological Transport; D015153:Blotting, Western; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D045744:Cell Line, Tumor; D002642:Chick Embryo; D002645:Chickens / Q000235:genetics* / Q000378:metabolism; D015243:Cholesterol, VLDL / Q000378:metabolism; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000235:genetics / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000235:genetics / Q000378:metabolism; D004967:Estrogens / Q000494:pharmacology*; D004997:Ethinyl Estradiol / Q000031:analogs & derivatives / Q000494:pharmacology; D005260:Female; D005347:Fibroblasts / Q000166:cytology / Q000378:metabolism; D019084:Fluorescent Antibody Technique, Indirect; D005786:Gene Expression Regulation; D015723:Gene Library; D008099:Liver / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D008297:Male; D008969:Molecular Sequence Data; D017510:Protein Folding; D021382:Protein Sorting Signals; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid
chemical_list: D001055:Apolipoproteins B; D002352:Carrier Proteins; D015243:Cholesterol, VLDL; D018076:DNA, Complementary; D004967:Estrogens; D021382:Protein Sorting Signals; D012333:RNA, Messenger; C106092:microsomal triglyceride transfer protein; D004997:Ethinyl Estradiol; C011310:moxestrol