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The cytostatic and cytolytic action of 22R - hydroxydesmosterol on hepatoma cells cultured in a medium containing 10% newborn-calf serum can be reversed within certain concentration limits by adding cholesterol to the culture medium. In contrast, under the same conditions, the cytotoxicity of 7 beta -hydroxycholesterol could not be reversed, whatever the concentrations of cholesterol added. However, in a lipoprotein-poor and in a chemically defined medium, the cytolytic action of both hydroxysterols can be reversed by adding cholesterol, but growth inhibition cannot be suppressed. This demonstrates the importance of serum lipids and lipoproteins for the toxicity of the hydroxysterols and for the antagonistic effect of cholesterol. Our results suggest that the action mechanisms of 7 beta-hydroxycholesterol and 22R - hydroxydesmosterol on HTC hepatoma cells are not fully identical.

3-hydroxy-3-methylglutaryl-coenzyme A reductase (EC 1.1.1.3.4.) activity and cell membrane fluidity measured by fluorescence polarization using 1,6 diphenyl, 1,3,5-hexatriene as fluorescent probe have been concomitantly examined in HTC hepatoma cells, both in relation to growth rate and in response to treatment with hydroxylated sterols. A high level of HMG-CoA reductase activity was observed in cells at log phase of growth which progressively decreased to reach a sustained low level at stationary phase. Similarly, membrane fluidity markedly decreased in relation to growth rate. Hydroxylated sterols such as 7 beta-hydroxycholesterol or 25-hydroxycholesterol strongly inhibited HMG-CoA reductase activity whereas a water-soluble derivative of 7 beta-hydroxycholesterol sodium 3,7-bishemisuccinate had no effect. Within the same range of concentrations 7 beta-hydroxycholesterol and 25-hydroxycholesterol strongly decreased membrane fluidity when the water-soluble derivative was ineffective. Thus, the present results provide evidence for a correlation between the two tested parameters and suggest a dependency of HMG-CoA reductase activity on cell membrane fluidity.

The toxicity of 7 beta-hydroxycholesterol for cultured HTC cells is 10 times greater if serum lipids and lipoproteins are absent from the culture medium. A water-soluble derivative of 7 beta-hydroxycholesterol, sodium 3,7-bishemisuccinate, showed the same toxicity as the original molecule and was also 8 times more toxic when serum lipids and lipoproteins were absent. But the rapid inhibition of DNA synthesis was similar in cells treated with both compounds, whether lipids and lipoproteins are present or not. Thus the absence of serum lipids and lipoproteins enhances the lytic effect of both substances but does not increase their intracellular action on DNA synthesis. This first parallel study on lipophilic 7 beta-hydroxycholesterol and its water-soluble homologue shows the importance of the serum lipids and lipoproteins in the cytotoxicity of such sterols.