The triple threat to nascent apolipoprotein B. Evidence for multiple, distinct degradative pathways.
We previously showed that Omega-3 fatty acids reduce secretion of apolipoprotein B (apoB) from cultured hepatocytes by stimulating post-translational degradation. In this report, we now characterize this process, particularly in regard to the two known processes that degrade newly synthesized apoB, endoplasmic reticulum (ER)-associated degradation and re-uptake from the cell surface. First, we found that Omega-3-induced degradation preferentially reduces the secretion of large, assembled apoB-lipoprotein particles, and apoB polypeptide length is not a determinant. Second, based on several experimental approaches, ER-associated degradation is not involved. Third, re-uptake, the only process known to destroy fully assembled nascent lipoproteins, was clearly active in primary hepatocytes, but Omega-3-induced degradation of apoB continued even when re-uptake was blocked. Cell fractionation showed that Omega-3 fatty acids induced a striking loss of apoB100 from the Golgi, while sparing apoB100 in the ER, indicating a post-ER process. To determine the signaling involved, we used wortmannin, a phosphatidylinositol 3-kinase (PI3K) inhibitor, which blocked most, if not all, of the Omega-3 fatty acid effect. Therefore, nascent apoB is subject to ER-associated degradation, re-uptake, and a third distinct degradative pathway that appears to target lipoproteins after considerable assembly and involves a post-ER compartment and PI3K signaling. Physiologic, pathophysiologic, and pharmacologic regulation of net apoB secretion may involve alterations in any of these three degradative steps.
Fisher
E A
EA
Laboratory of Lipoprotein Research, The Zena and Michael A. Wiener Cardiovascular Institute and Department of Medicine, Mount Sinai School of Medicine, New York, New York 10029, USA.
Pan
M
M
Chen
X
X
Wu
X
X
Wang
H
H
Jamil
H
H
Sparks
J D
JD
Williams
K J
KJ
D000730:Androstadienes / Q000494:pharmacology; D000818:Animals; D001055:Apolipoproteins B / Q000737:chemistry* / Q000378:metabolism*; D002478:Cells, Cultured; D003546:Cysteine Endopeptidases / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D015525:Fatty Acids, Omega-3 / Q000378:metabolism; D006056:Golgi Apparatus / Q000378:metabolism; D022781:Hepatocytes / Q000378:metabolism; D008079:Lipoproteins, VLDL / Q000378:metabolism; D008297:Male; D009097:Multienzyme Complexes / Q000378:metabolism; D011233:Precipitin Tests; D046988:Proteasome Endopeptidase Complex; D051381:Rats; D017207:Rats, Sprague-Dawley; D015398:Signal Transduction; D013347:Subcellular Fractions / Q000378:metabolism; D014280:Triglycerides / Q000378:metabolism; D000077191:Wortmannin
D000730:Androstadienes; D001055:Apolipoproteins B; D004791:Enzyme Inhibitors; D015525:Fatty Acids, Omega-3; D008079:Lipoproteins, VLDL; D009097:Multienzyme Complexes; D014280:Triglycerides; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D000077191:Wortmannin