Endogenous metabolites of the mevalonate/chotesterol/bile acid biosynthetic pathway function as intracrine regulators of hepatocyte physiology, which act by modifying the activities of orphan nuclear receptors. An anti-cholesterol drug that inhibits a key step in this pathway will cause the accumulation of one of these bioactive molecules and activate the pleiotropic responses associated with the mediating nuclear receptors, which include induction of cytochromes P450. Our hypothesis is that (1) of the anti-cholesterol drugs that induce hepatic P450s, only a subset (e.g., certain statins and the squalene monooxygenase inhibitor, NB-598) are able to bind directly to a xenobiotic-sensing nuclear receptor, and these do so in a species-specific manner; (2) squalestatin 1 induces rat CYP2B (and activates CAR) indirectly, either by causing the accumulation of a farnesoid, which functions as a CAR ligand, or by provoking a disturbance in heme metabolism; the bioactive mediator of this effect is metabolized or eliminated from the hepatocyte by a pregnenolone 16(-carbonitrile (PCN)-inducible enzyme or transporter; and (3) multiple classes of endogenous metabolites of the mevalonate/sterol/bile acid pathway that accumulate in the human hepatocyte following anti-cholesterol drug treatment and/or mevalonate supplementation are capable of inducing CYP3A and CYP2B6 expression as a consequence of PXR binding and activation.
The specific aims are (1) To define the abilities of anti-cholesterol drugs to interact directly with the rat, mouse, and human CAR, PXR, and PPARalpha receptors. (2) To identify the specific branch of the mevalonate pathway that mediates squalestatin 1-inducible CYP2B expression in primary cultured rat hepatocytes. (3) To identify the mechanism that is responsible for PCN-mediated, mevalonate-reversible, suppression of squalestatin 1-inducible CYP2B expression in primary cultured rat hepatocytes, and to confirm that the mechanisms governing squalestatin 1-mediated CYP2B induction that have been defined in primary cultured rat hepatocytes are also operative in vivo. (4) To identify the specific metabolic intermediates of the mevalonate/sterol/bile acid biosynthetic pathway that regulate CYP3A and CYP2B6 expression in human hepatocyte models. These studies will provide essential new information about the pharmacology of a widely-used class of drugs, and important insights into the mechanisms underlying inducible P450 expression.
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