To Örebro University

BACKGROUND AND PURPOSE: Many drugs and environmental contaminants induce hypercholesterolemia and promote the risk of atherosclerotic cardiovascular disease. The mechanisms involved are poorly defined precluding efficient prediction and prevention. We tested the hypothesis that pregnane X receptor (PXR), a xenobiotic-sensing nuclear receptor, regulates the level of circulating atherogenic lipids in humans and utilized mouse experiments to identify the mechanisms involved.

EXPERIMENTAL APPROACH: We performed serum NMR metabolomics in healthy volunteers administered rifampicin, a prototypical human PXR ligand, or placebo in a crossover setting. Furthermore, we used high-fat diet fed wildtype and PXR knockout mice to investigate the mechanisms and pathways mediating the PXR-induced alterations in cholesterol homeostasis.

KEY RESULTS: Activation of PXR induced cholesterogenesis both in pre-clinical and clinical settings. In human volunteers, rifampicin increased IDL, LDL and total cholesterol and lathosterol-cholesterol ratio, a marker of cholesterol synthesis, suggesting increased cholesterol synthesis. Mechanistic studies in mice indicated that PXR activation launches widespread induction of the cholesterol synthesis genes including the rate-limiting Hmgcr and upregulates the intermediates in the Kandutsch-Russell cholesterol synthesis pathway in the liver. Additionally, PXR activation induced plasma PCSK9, a negative regulator of hepatic LDL uptake, in both mice and humans. We propose that these effects were mediated through increased proteolytic activation of SREBP2 in response to PXR activation.

CONCLUSION AND IMPLICATIONS: PXR activation induces cholesterol synthesis and elevates LDL and total cholesterol in humans. The PXR-SREBP2 pathway is a novel regulator of the cholesterol and PCSK9 synthesis, and a molecular mechanism for drug- and chemical-induced hypercholesterolemia.