Inhibition of cholesterol synthesis and cell growth by 24(R,S),25-iminolanosterol and triparanol in cultured rat hepatoma cells
24(R,S),25-Iminolanosterol (IL) and triparanol disrupt cholesterol biosynthesis in rat hepatoma H4-II-C3 (H4) cells by inhibiting the conversion of lanosterol to cholesterol. Depending on concentration, these inhibitors lead to the intracellular accumulation of sterol intermediates. At 45 μM, both compounds promote the buildup of zymosterol (5α-cholesta-8(9),24-dien-3β-ol), while at a lower concentration of 4.5 μM, they cause accumulation of desmosterol (cholesta-5,24-dien-3β-ol). Intermediate concentrations (9 or 22.5 μM) result in the accumulation of three sterols: cholesta-5,7,24-trien-3β-ol, zymosterol, and desmosterol.
These intermediary sterols, normally absent in H4 cells, are especially prominent when cells are cultured in lipid-depleted media containing the inhibitors, indicating synthesis from endogenous precursors at the expense of cholesterol production. Both IL and triparanol completely block the synthesis of cholesterol from [¹⁴C]acetate and [2-¹⁴C]mevalonate, even at the lowest tested concentration (4.5 μM).
In addition to disrupting sterol synthesis, IL and triparanol inhibit H4 cell proliferation. Cells exposed to 22.5 μM IL in either full-growth or lipid-depleted medium fail to proliferate unless supplemented with low-density lipoproteins (LDL, 60 μg/mL). Notably, supplementation with 4.6 mM mevalonate does not rescue the inhibitory effect of IL on cell growth.