FXR and LXR Regulation of Cholesterol Metabolism
Cholesterol is essential for life and a key in the development of heart disease. Cholesterol homeostasis is achieved through regulation of cholesterol uptake, cholesterol biosynthesis, cholesterol conversion to bile acids and excretion of bile acids. Inhibition of cholesterol biosynthesis upregulates LDLR expression and is the mechanism of action of many drugs used to lower plasma LDL to reduce coronary heart disease. Many aspects of cholesterol homeostasis are regulated by the nuclear receptors FXR and LXR, both nuclear receptor transcription factors that form heterodimers with the retinoic acid RXR receptors and that are activated by cholesterol metabolites. One of the primary tissues in cholesterol metabolism is the liver, a key site of cholesterol biosynthesis and where cholesterol low-density lipoprotein (LDL) is taken up from the plasma by the LDL-receptor. When cholesterol accumulates in liver cells, some of the cholesterol is oxidized to create oxysterols. Oxysterols activate LXR through LXR/RXR heterodimers to activate genes such as the CYP7A1 enzyme that catalyzes the rate-limiting step in bile acid biosynthesis and a major route for the elimination of cholesterol. Animals lacking the CYP7A1 enzyme accumulate cholesterol in the liver. In the intestine LXR activates the ABC-1 gene, a transporter that actively transports cholesterol out of cells to clear it from the body. Activation of ABC-1 expression by LXR in macrophages in atherosclerotic plaques appears to be another mechanism by which LXR plays a role in heart disease. The FXR receptor is activated by bile acids. In the liver, activation of FXR-RXR heterodimers by bile acids results in the feedback inhibition of CYP7A expression and reduced biosynthesis of bile acids. In the intestine, FXR activates expression of I-BABP, a protein that increases the transport of bile acids back to the liver from the intestine, reducing their excretion. Drugs targeting the FXR and LXR receptors could play an important role in modulating cholesterol homeostasis and heart disease in the future.
Contributor: Glenn Croston, PhD.
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