A targeted apolipoprotein B-38.9-producing mutation causes fatty livers in mice due to the reduced ability of apolipoprotein B-38.9 to transport triglycerides.
Nonphysiological truncations of apolipoprotein (apo) B-100 cause familial hypobetalipoproteinemia (FHBL) in humans and mice. An elucidation of the mechanisms underlying the FHBL phenotypes may provide valuable information on the metabolism of apo B-containing lipoproteins and the structure-function relationship of apo B. To generate a faithful mouse model of human FHBL, a subtle mutation was introduced into the mouse apo B gene by targeting embryonic stem cells using homologous recombination followed by removal of the selection marker gene by Cre-loxP-mediated site-specific recombination. The engineered mice bear a premature stop codon at residue 1767 and a 42-base pair loxP inserted into intron 24 of the apo B gene, thus closely resembling the apo B-38.9-producing mutation in humans. Apo B-38.9 was the sole apo B protein in homozygote (apob(38.9/38.9)) plasma. In heterozygotes (apob(+/)(38. 9)), apo B-100 and apo B-48 were reduced by 75 and 40%, respectively, and apo B-38.9 represented 20% of total circulating apo B. Hepatic apo B-38.9 mRNA levels were reduced by 40%. In cultured apob(+/)(38. 9) hepatocytes, apo B-100 was produced in trace quantities, and the synthesis rate of apo B-38.9 relative to apo B-48 was reduced by 40%. However, almost equimolar amounts of apo B-38.9 and apo B-48 were secreted into the media. Pulse-chase studies revealed that apo B-38. 9 was secreted at a faster rate and more efficiently than apoB-48. Nevertheless, both apob(+/)(38.9) and apob(38.9/38.9) mice had reduced hepatic triglyceride secretion rates and fatty livers. Thus, low mRNA levels or defective secretion of apo B-38.9 may not be responsible for the FHBL phenotypes caused by the apo B-38.9 mutation. Rather, a reduced capacity of apo B-38.9 for triglyceride transport may account for the fatty livers in these mice.
Chen
Z
Z
Division of Atherosclerosis, Nutrition and Lipid Research, Department of Medicine, Washington University School of Medicine, St. Louis, Missouri 63110, USA. zchen@im.wustl.edu
Fitzgerald
R L
RL
Averna
M R
MR
Schonfeld
G
G
D000818:Animals; D053299:Apolipoprotein B-100; D001055:Apolipoproteins B / Q000096:biosynthesis / Q000097:blood / Q000235:genetics* / Q000378:metabolism; D001483:Base Sequence; D002478:Cells, Cultured; D002784:Cholesterol / Q000097:blood; D005234:Fatty Liver / Q000235:genetics* / Q000378:metabolism; D022781:Hepatocytes / Q000166:cytology / Q000378:metabolism*; D006801:Humans; D006995:Hypobetalipoproteinemias / Q000235:genetics; D007421:Intestine, Small / Q000378:metabolism; D008099:Liver / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D008815:Mice, Inbred Strains; D008822:Mice, Transgenic; D010743:Phospholipids / Q000097:blood; D011092:Polyethylene Glycols / Q000494:pharmacology; D016133:Polymerase Chain Reaction; D016415:Sequence Alignment; D012689:Sequence Homology, Nucleic Acid; D013501:Surface-Active Agents / Q000494:pharmacology; D014280:Triglycerides / Q000097:blood*
C415826:Apob protein, mouse; D053299:Apolipoprotein B-100; D001055:Apolipoproteins B; D010743:Phospholipids; D013501:Surface-Active Agents; D014280:Triglycerides; D011092:Polyethylene Glycols; D002784:Cholesterol; C016811:tyloxapol