INHIBITION OF CHOLESTEROL SYNTHESIS BY POLICOSANOL

Cholesterol is an essential component of the cell, but excessive blood levels are a major risk factor for the development of atherosclerotic plaques that can lead to heart disease and stroke, the foremost cause of premature death in Western societies. Policosanol, a mixture of very long chain alcohols derived from sugarcane, has gained considerable attention among the public as safe and effective means to reduce blood cholesterol levels, a belief based on some early clinical studies. My research investigates one possible mechanism by which policosanol might decrease blood cholesterol levels: the inhibition of cholesterol synthesis in the liver. Previous studies with cultured hepatoma cells have indicated that policosanol suppresses HMG-CoA reductase activity, the regulatory step in cholesterol synthesis, by activation of AMP-kinase, which then inactivates HMG-CoA reductase by phosphorylation. My studies have confirmed this activation of AMP-kinase both in hepatoma cells and in whole animals after intragastric administration of policosanol. The present studies were also undertaken to identify the upstream signaling mechanism by which policosanol activates AMP-kinase. Treatment of rat hepatoma cells with policosanol increased the amount of phosphorylated CaMKK, which can directly activate AMP-kinase, but had only a small effect on LKB1, the principal activator of AMP-kinase. Intragastric administration to mice similarly activated CaMKK, but not LKB1, in the liver. To determine if metabolism of policosanol was necessary for activation of AMP-kinase, siRNA-mediated suppression of fatty aldehyde dehydrogenase, fatty acyl CoA synthase-4, or β-ketothiolase in hepatoma cells prevented the phosphorylation of AMP-kinase and HMG-CoA reductase by policosanol, indicating that metabolism of these very long chain alcohols to fatty acids and subsequent peroxisomal β-oxidation is necessary for the suppression of cholesterol synthesis. As the principal product of fatty acid -oxidation is acetyl-CoA, further studies demonstrated that addition of acetate to cells similarly activated AMP-kinase and inactivated HMG-CoA reductase. This finding argues that the activation of AMP-kinase by policosanol results from the generation of excess acetyl-CoA via peroxisomal metabolism. Finally, although the intestine is a significant source of circulating cholesterol, policosanol was unable to activate AMP-kinase in the small intestine. These findings open new perspectives for the control of cholesterol synthesis by activators of AMP-kinase