Movement of apolipoprotein B into the lumen of microsomes from hepatocytes is disrupted in membranes enriched in phosphatidylmonomethylethanolamine
Rusinol, A.E. | Chan, E.Y.W. | Vance, J.E.
When monolayer cultures of rat hepatocytes are incubated with the ethanolamine/choline analogue, monomethylethanolamine, the secretion of apolipoproteins B100 and B48, as well as the lipid constituents, of very low density lipoprotein (VLDL) is inhibited by approximately 50% (Vance, J.E. (1991) J. Lipid Res. 32, 1971-1982). In the present study we have investigated the mechanism by which monomethylethanolamine disrupts VLDL secretion. Hepatocytes were treated with 400 micromolar monomethylethanolamine overnight, which resulted in an increase in the cellular content of the derived phospholipid, phosphatidylmonomethylethanolamine, from 0.32 +/- 0.15 to 2.92 +/- 0.74 nmol/mg of cell protein. The biosynthesis of apoproteins B100 and B48 was not impaired by treatment of cells with monomethylethanolamine. However, monomethylethanolamine decreased by approximately 50% the amount of apoproteins B, but not of the typical secretory protein, albumin, present in the luminal content subfraction of microsomes. The intracellular degradation of apoproteins B was also increased in phosphatidylmonomethylethanolamine-enriched, compared with control, cells. Moreover, the pool of apoprotein B present in intact microsomes from hepatocytes incubated with monomethylethanolamine was more accessible to exogenously added trypsin, presumably because a larger pool of the apoprotein B was exposed on the cytosolic surface of these microsomes. The data strongly suggest that an increase in the microsomal content of phosphatidylmonomethylethanolamine diminishes the ability of apoprotein B to translocate across the endoplasmic reticulum membrane into the luminal compartment. Consequently, the association of apoprotein B with lipids and/or the normal assembly of mature VLDL particles is impaired.
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