Endogenous metabolites of the cholesterol biosynthetic pathway are now recognized to function not only as chemical intermediates, but also as intracrine regulators of hepatocyte physiology, which modify the activities of nuclear receptors. Recently, many "anti-cholesterol drugs" have been shown to cause bioactive metabolites to accumulate in the liver, resulting in altered hepatic gene expression. For example, we found that inhibitors of squalene synthase (e.g., squalestatin 1) selectively induce cytochrome P450 2B (CYP2B) gene expression by causing an endogenous farnesoid to accumulate, which activates the constitutive androstane receptor (CAR). We propose that the biological activities of the farnesoids contribute to the therapeutic as well as the toxic effects of squalene synthase inhibitors. We hypothesize that farnesoid accumulation mediated by squalene synthase inhibition causes distinctive changes in hepatocellular gene expression that are conserved from rodent to human and from primary culture to in vivo. Many of these changes (e.g., CYP2B induction) are attributable to farnesol accumulation and CAR activation, and these changes are enhanced by inhibition of alcohol dehydrogenase (ADH)-catalyzed farnesol metabolism. Other changes (e.g., Slc13A3 induction) are mediated by a farnesol-derived metabolite(s) and a CAR-independent mechanism(s).
The specific aims of this proposal are (1) to define and compare (i.e., among rat, mouse, and human) the global gene expression response of the hepatocyte to squalene synthase inhibition, and to determine whether CAR mediates the components of this response, (2) to determine whether the effects of squalene synthase inhibition on human hepatocellular gene expression that we identify in primary cultured human hepatocytes (from Aim 1) are maintained in vivo, (3) to determine whether one or more ADH enzymes are responsible for converting farnesol to farnesal in hepatocytes, and hence whether ADH activity is a critical determinant of the farnesoid levels and effects on gene expression that are achieved following squalene synthase inhibition, and (4) to determine whether squalene synthase inhibition modulates the activities of individual human CAR isoforms. Endogenous farnesoid metabolism and signaling in normal human hepatocytes has been essentially unexplored. These studies will provide new information about the abilities of farnesoids to produce beneficial effects on hepatocellular lipid metabolism, as well as effects that promote hepatotoxicity. This information will be directly relevant to the future role of squalene synthase inhibitors as anti-cholesterol drugs.
Coronary heart disease is the leading cause of mortality in the United States, causing more deaths annually than all forms of cancer combined, and elevated low-density lipoprotein levels are a major risk factor for the development of coronary heart disease. Anti-cholesterol drug therapy, using statins, is now a proven approach for reducing the risk of developing coronary heart disease, and since behaviors that promote hypercholesterolemia are prevalent, statins are consumed by many people. However, certain patients do not respond to statin treatment and others cannot take the drugs due to the development of severe adverse drug reactions. Therefore alternative anti-cholesterol drugs are needed. The public health relevance of this project is that it will increase our knowledge about the potential utility of a novel class of anti-cholesterol drugs.
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