The use of stable and radioactive sterol tracers as a tool to investigate cholesterol degradation to bile acids in humans in vivo

Abstract

Alterations of cholesterol homeostasis represent important risk factors for atherosclerosis and cardiovascular disease. Different clinical-experimental approaches have been devised to study the metabolism of cholesterol and particularly the synthesis of bile acids, its main catabolic products. Most evidence in humans has derived from studies utilizing the administration of labeled sterols; these have several advantages over in vitro assay of enzyme activity and expression, requiring an invasive procedure such as a liver biopsy, or the determination of fecal sterols, which is cumbersome and not commonly available. Pioneering evidence with administration of radioactive sterol derivatives has allowed to characterize the alterations of cholesterol metabolism and degradation in different situations, including spontaneous disease conditions, aging, and drug treatment. Along with the classical isotope dilution methodology, other approaches were proposed, among which isotope release following radioactive substrate administration. More recently, stable isotope studies have allowed to overcome radioactivity exposure. Isotope enrichment studies during tracer infusion has allowed to characterize changes in the degradation of cholesterol via the "classical" and the "alternative" pathways of bile acid synthesis. Evidence brought by tracer studies in vivo, summarized here, provides an exceptional tool for the investigation of sterol metabolism, and integrate the studies in vitro on human tissue.

Scheme 1

Schematic illustration of the main pathways of bile acid synthesis from cholesterol.

Scheme 2

Illustration of the main metabolic steps of the classical pathway of bile acid synthesis. 1: Cholesterol-7α-hydroxylase (CYP7A); 2: 3β-hydroxy-Δ⁵-C27-steroid dehydrogenase (C₂₇β-HSD); 3: Sterol 12α-hydroxylase (CYP8B1); 4: Δ⁴-3-oxosteroid-5β-reductase; 5: 3α-hydroxysteroid dehydrogenase.