USE OF CYCLODEXTRIN TO DELIVER LIPIDS AND TO MODULATE APOLIPOPROTEIN-B-100 PRODUCTION IN HEPG2-CELLS

2-Hydroxypopyl-beta-cyclodextrin (cyclodextrin), cyclodextrin solubili zed oleate, and cyclodextrin-solubilized cholesterol were used to modu late proteolysis and secretion of newly-synthesized apolipoprotein B-1 00 (apoB) in HepG2 cells. Following cyclodextrin and lipid treatments, cells were pulse-labeled with [H-3] leucine, and quantitative immunop recipitation was used to measure apoB synthesis, apoB secreted into th e medium, and the cellular content of undegraded apoB that was not sec reted. Three-hour treatment with cyclodextrin-solubilized oleate (0.2 mM) increased secreted apoB from 4% (control cells) to 32% and cellula r undegraded apoB from 15% (control cells) to 64% of apoB synthesized, which is consistent with earlier studies using bovine serum albumin t o complex exogenous oleate. Prolonged daily (4 d or more) administrati on of 0.5% (3.5 mM) cyclodextrin with medium containing 10% fetal bovi ne serum increased the secretion of nascent apoB from 5-10% (control) to 17-28% and cellular undegraded apoB from 15-20% (control) to 25-31% of apoB synthesized, respectively. Subsequent administration of cyclo dextrin-solubilized cholesterol (10-40 mu g) for only 3 h reversed the cyclodextrin-mediated increase in apoB secretion. The application of 0.5% cyclodextrin to HepG2 cells can rapidly (within minutes) stimulat e cholesterol efflux, and transiently (over a 1-2 d period) increase c holesterol synthesis. In the current studies, the cyclodextrin-mediate d increase in cholesterol synthesis was not concurrent with the increa se in apoB secretion. However, prolonged (15 d) administration of cycl odextrin was shown to increase the cellular free cholesterol concentra tion by 25-41%, reduce the cellular triglyceride concentration by 59%, and increase apoB secretion 3- to 4-fold, without affecting the cellu lar cholesteryl ester concentration. In comparison, 14-d treatment wit h cyclodextrin-solubilized cholesterol (20 mu g/mL) followed by 1-d eq uilibration without cholesterol was shown to increase the cellular fre e cholesterol and cholesteryl ester concentrations by 76% and 10-fold, respectively, although apoB secretion was not affected. It is hypothe sized that chronic daily administration of 0.5% cyclodextrin increased the cellular cholesterol concentration and flux in discrete putative regulatory compartments, which ''shielded'' nascent apoB from rapid pr oteolysis and facilitated apoB secretion. In conclusion, cyclodextrin was used independently and in combination with cholesterol or oleate t o modulate apoB proteolysis and secretion. We speculate that subcellul ar changes in cholesterol concentration and flux may modulate apoB pro duction in HepG2 cells.