Characterization of a mutant form of human apolipoprotein B (Thr26_Tyr27del) associated with familial hypobetalipoproteinemia

We have previously identified a deletion mutant of human apoB [apoB (Thr26_Tyr27del)] in a subject with primary
hypobetalipoproteinemia. The present study determined the effect of Thr26_Tyr27del mutation on apoB
secretion using transfected McA-RH7777 cells. Transient or stable transfection of apoB-48 containing the
Thr26_Tyr27del mutation showed drastically reduced secretion of the mutant as compared to wild-type apoB-
48. No lipoproteins containing the mutant apoB-48 were secreted into the medium. Incubation of transfected
cells in a lipid-rich medium in the presence of cycloheximide showed rapid turnover of cell-associated mutant
apoB-48 as compared to that of wild-type apoB-48. Immunofluorescence experiments showed that the mutant
apoB-48 was mostly localized in the endoplasmic reticulum. Treatment with the proteasomal inhibitor MG132
markedly attenuated the turnover of cell-associated mutant apoB-48, whereas treatment with inhibitors of
autophagosomal/lysosomal function (e.g. 3-MA or ammonium chloride) had no effect. Taken together, these results
indicated that the defective secretion of the Thr26_Tyr27delmutantwas associated with increased intracellular
degradation of apoB through the proteasome-dependent pathway.