The liver is the major organ responsible for removing LDL from plasma, a process which regulates plasma LDL levels. Plasma LDL levels is one of the most important risk factors associated with atherosclerosis. While it is well-documented that expression of LDL receptors is down-regulated by cholesterol, the identity and metabolic processes involved are unknown. Using a several different experimental models, we have shown that 7-alpha-hydroxylase metabolizes oxysterols to inactive bile acid intermediates. As a result, hepatic expression of 7-alpha-hydroxylase, the liver shows a unique resistance to repression of gene expression. Based on this rationale, we have partially purified natural products that show the ability to repress the transcription of the luciferase driven by the HMG-CoA reductase promoter and to induces the expression of 7- alpha-hydroxylase mRNA in a novel line of hepatoma cells displaying an adult liver phenotype. The major goal of this research is to identify these natural products and examine their biosynthesis, metabolism and mechanism of action to achieve the following specific aims: (1) to identify the natural products responsible for repressing the transcription of the LDL receptor, (2) to determine the biosynthetic and catabolic pathways responsible for the synthesis and catabolism (inactivation) of natural product repressors, (3) to identify the natural products responsible for the induction of 7-alpha-hydroxylase by LDL and its metabolism by the liver, (4) to determine the mechanism through which these natural products increase the relative abundance of 7-alpha- hydroxylase, and, (5) to identify the natural product binding protein. The knowledge gain from these proposed studies will provide new insights in developing pharmacologic agents to control the expression of the cholesterol-repressible genes and 7-alpha-hydroxylase.
