COTRANSLATIONAL INHIBITION OF APO-B-100 SYNTHESIS BY CYCLOSPORINE-A IN THE HUMAN HEPATOMA-CELL LINE HEPG2

Treatment of patients with cyclosporin A (CsA) increases low-density l ipoprotein (LDL) cholesterol levels. We investigated whether an elevat ed hepatic secretion of apolipoprotein (apo) B-100-containing lipoprot eins is responsible for the increase of LDL by using the human hepatom a cell line HepGZ. Addition of CsA to the culture medium of HepG2 cell s resulted in a dose- and time-dependent decrease in the secretion of apoB-100. Maximal inhibition (-50%), which was obtained at 5 mu mol/L CsA, was achieved within 8 hours. The secretion of apoA-I, albumin, an d [S-35]methionine-labeled proteins was not affected by CsA. The reduc ed accumulation of apoB-100 in the culture medium could not be explain ed by changes in the uptake and degradation of LDL by HepG2 cells trea ted with CsA. In addition, [S-35]methionine incorporation studies indi cated that synthesis and/or secretion of newly synthesized apoB-100 de creased in the presence of CsA. CsA did not affect the apoB-100 mRNA l evel, indicating that CsA regulates the secretion of apoB-100 at the c otranslational or posttranslational level. The decreased secretion of apoB-100 was accompanied by a diminished secretion of triglycerides (- 47%), cholesterol (-18%), and cholesteryl esters (-27%) in the presenc e of CsA. In contrast, the intracellular concentrations and the total amount of these lipids present in the culture medium and cells were no t changed. This indicates that a possible limited availability of one of these lipids was not responsible for the decreased secretion of apo B-100 by CsA. Pulse-chase experiments showed that the amount of intrac ellular apoB-100 was already decreased by 50% after the 10-minute puls e period and that CsA did not affect the intracellular processing of a poB-100 once it was fully synthesized. Short pulse incubations in the presence of [S-35]methhonine showed a decrease in the intracellular am ount of labeled apoB-100 after an incubation of only 2 through 4 minut es, indicating that the translation was not affected but that inhibiti on of the apoB-100 secretion by CsA occurred at the cotranslational le vel. Our results suggest that the elevated plasma LDL levels observed in patients treated with CsA are not caused by hepatic overproduction of apoB-100-containing lipoproteins.