PRAVASTATIN INHIBITED THE CHOLESTEROL-SYNTHESIS IN HUMAN HEPATOMA-CELL LINE HEP G2 LESS-THAN SIMVASTATIN AND LOVASTATIN, WHICH IS REFLECTEDIN THE UP-REGULATION OF 3-HYDROXY-3-METHYLGLUTARYL COENZYME-A REDUCTASE AND SQUALENE SYNTHASE

The possible difference between lovastatin (mevinolin, MK-803), simvas tatin (MK-733) and pravastatin (CS-514), all chemically-related compet itive inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) re ductase, were tested in the human hepatoma cell line Hep G2, which is often used as a model for the human hepatocyte. After an 18-hr incubat ion of the cells with the drugs, pravastatin (IC50 = 1900 nM) was less potent than simvastatin and lovastastin (IC50 = 34 and 24 nM, respect ively) in inhibiting the sterol synthesis. As a consequence of this in hibition, the HMG-CoA reductase mRNA levels and squalene synthase acti vity, both negatively-regulated by sterols, were increased equally by simvastatin and lovastatin, whereas the induction by pravastatin was m uch less. In contrast, there were fewer differences between the compou nds in inhibiting HMG-CoA reductase activity, when assayed directly in Hep G2 cell homogenates (IC50 values = 18, 61 and 95 nM for simvastat in, lovastastin and pravastatin, respectively). Moreover, in experimen ts with human hepatocytes in primary culture the IC50 values for inhib ition of the cholesterol synthesis by simvastatin and pravastatin were of the same order of magnitude (23 and 105 nM, respectively). The res ults are therefore explained as follows: the three drugs act in the sa me way within the Hep G2 cell in terms of inhibiting HMG-CoA reductase and their subsequent effect on the feedback regulation of the cholest erol synthesis, i.e. increasing squalene synthase and HMG-CoA reductas e mRNA. However, pravastatin seems to be less able to enter the cells compared with simvastatin and lovastatin, possibly because of the high er hydrophobicity of the latter compounds. The observation with human hepatocytes suggests that in Hep G2 cells a specific hepatic transport er is missing. On one hand the human hepatoma cell line Hep G2 has pro ved to be a good model for the study of the feedback regulation of enz ymes of the cholesterol biosynthetic pathway such as HMG-CoA reductase and squalene synthase, but, on the other hand seems to be less suitab le as a model for the study of specific uptake of drugs, e.g. the vast atins, in human hepatocytes.