Lipoprotein Assembly: A Potential Target for Drugs Affecting Lipid Metabolism

Abstract

The human recessive disorder abetalipoproteinemia is associated with a complete inability to assemble and secrete apo B-containing lipoproteins by both the liver and intestine. The gene responsible for this disease does not co-segregate with the apo a gene suggesting it is caused by the loss of some gene product. To gain an understanding of the apparently unique process necessary for apo a secretion, plasmids expressing apo a were transfected into Chinese hamster ovary fibroblasts. Expression of a truncated form (apo B15, consisting of 15% of the N-terminus of the full length protein), which is too short to assemble lipoproteins, resulted in the accumulation of apo a in both cells and culture medium. In contrast, cells transfected with a plasmid encoding apo B53 (which is sufficiently large to allow the assembly and secretion of lipoproteins when expressed in hepatoma cells) did not contain apo a nor did they secrete it despite the presence of apo a mRNA. Incubation with calpain inhibitor I (ALLN) led to the accumulation of apo B53 in cells, showing that it is synthesized, but completely degraded in the absence of the inhibitor. Despite the accumulation of apo B53 in ALLN-treated cells, none was secreted. The inability of Chinese hamster ovary cells to secrete apo B53 cannot be explained by insufficient lipid since stimulation of triglyceride synthesis did not induce secretion. Essentially all of the apo B53 which accumulated in the microsomes from ALLN-treated cells was bound to microsomal membranes and susceptible to degradation by exogenous trypsin, while the lumenal enzyme protein disulfide isomerase was fully protected. These data suggest that translocation is the step in the secretory pathway responsible for sorting apo a into either the secretory or degradation (i.e. default) pathways. Moreover, it appears that apo B requires a unique process, not expressed in Chinese hamster ovary cells, for its translocation. This process required for translocation may be lacking in abetalipoproteinemia. Based on the findings that essentially no toxicity is associated with either the abetalipoproteinemic phenotype or the phenotype created using fibroblast cells reported here, we propose that agents that would create the abetalipoproteinemic phenotype would be useful in treating hyperlipidemic conditions associated with increased plasma apo a levels.

*

Work supported by NIH grant HL 41624