apoA-IV tagged with the ER retention signal KDEL perturbs the intracellular trafficking and secretion of apoB.
To examine the role of apolipoprotein A-IV (apoA-IV) in the intracellular trafficking and secretion of apoB, COS cells were cotransfected with microsomal triglyceride transfer protein (MTP), apoB-41 (amino terminal 41% of apoB), and either native apoA-IV or apoA-IV modified with the carboxy-terminal endoplasmic reticulum (ER) retention signal, KDEL (apoA-IV-KDEL). As expected, apoA-IV-KDEL was inefficiently secreted relative to native apoA-IV. Coexpression of apoB-41 with apoA-IV-KDEL reduced the secretion of apoB-41 by approximately 80%. The apoA-IV-KDEL effect was specific, as neither KDEL-modified forms of human serum albumin or apoA-I affected apoB-41 secretion. Similar results were observed in McA-RH7777 rat hepatoma cells, which express endogenous MTP. The full inhibitory effect of apoA-IV-KDEL on apoB secretion was observed only for forms of apoB containing a minimum of the amino-terminal 25% of the protein (apoB-25). However, apoA-IV-KDEL inhibited the secretion of both lipid-associated and lipid-poor forms of apoB-25. Dual-label immunofluorescence microscopy of cells transfected with native apoA-IV and apoB-25 revealed that both apolipoproteins were localized to the ER and Golgi, as expected. However, when apoA-IV-KDEL was cotransfected with apoB-25, both proteins localized primarily to the ER. These data suggest that apoA-IV may physically interact with apoB in the secretory pathway, perhaps reflecting a role in modulating the process of triglyceride-rich lipoprotein assembly and secretion.
Gallagher
James W
JW
Department of Pathology, Wake Forest University School of Medicine, Winston-Salem, NC 27157, USA.
Weinberg
Richard B
RB
Shelness
Gregory S
GS
D000818:Animals; D001054:Apolipoproteins A / Q000235:genetics / Q000502:physiology*; D001055:Apolipoproteins B / Q000235:genetics / Q000378:metabolism*; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D002460:Cell Line; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D006056:Golgi Apparatus / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008055:Lipids; D021382:Protein Sorting Signals / Q000502:physiology*; D021381:Protein Transport; D051381:Rats; D014162:Transfection
D001054:Apolipoproteins A; D001055:Apolipoproteins B; D002352:Carrier Proteins; D008055:Lipids; D021382:Protein Sorting Signals; C022205:apolipoprotein A-IV; C106092:microsomal triglyceride transfer protein