Effects of gemfibrozil and ketoconazole on human apolipoprotein AI, B and E levels in two hepatoma cell lines, HepG2 and Hep3B.

Exposure of HepG2 and Hep3B cells to gemfibrozil (40 micrograms/ml), a hypolipidemic drug, resulted in a 2-fold induction of apo AI mRNA and, a one-third reduction in apo B mRNA but had no significant effect on apo E mRNA levels. The hypothesis that the mechanism of action of gemfibrozil involved the cytochrome P-450 system was tested by using ketoconazole, a P-450 inhibitor, which blocks the formation of endogenous polar sterols. When the cells were treated with ketoconazole at a concentration of greater than or equal to 15 microM, the levels of apo AI, and apo B mRNAs decreased by 50% and 35%, respectively, compared to the basal level. No significant effect on apo E mRNA level was observed during ketoconazole treatment. The effects of gemfibrozil and ketoconazole on various apolipoprotein secretion were studied using pulse-chase experiments. It was observed that the selective alterations in the rates of apo AI and apo B production were occurring at the level of synthesis. This observation is consistent with the findings indicating a strong direct correlation between hepatic apolipoprotein mRNA concentration and secreted apolipoprotein levels. The induction of apo AI mRNA by gemfibrozil was not apparent when the cells were simultaneously treated with ketoconazole. However, the level of apo B mRNA was reduced further to less than 55% of the control level suggesting that there might be an additive effect of these two drugs on apo B synthesis.