Increased hepatobiliary and fecal cholesterol excretion upon activation of the liver X receptor is independent of ABCA1

The ATP-binding cassette transporter ABCA1 is essential for high density lipoprotein (HDL) formation and considered rate-controlling for reverse cholesterol transport. Expression of the Abca1 gene is under control of the liver X receptor (LXR). We have evaluated effects of LXR activation by the synthetic agonist T0901317 on hepatic and intestinal cholesterol metabolism in C57BL/6J and DBA/1 wild-type mice and in ABCA1-deficient DBA/1 mice. In wild-type mice, T0901317 increased expression of Abca1 in liver and intestine, which was associated with a similar to60% rise in HDL. Biliary cholesterol excretion rose 2.7-fold upon treatment, and fecal neutral sterol output was increased by 150-300%. Plasma cholesterol levels also increased in treated Abca1(-/-) mice (+120%), but exclusively in very low density lipoprotein-sized fractions. Despite the absence of HDL, hepatobiliary cholesterol output was stimulated upon LXR activation in Abca1(-/-) mice, leading to a 250% increase in the biliary cholesterol/phospholipid ratio. Most importantly, fecal neutral sterol loss was induced to a similar extent (+300%) by the LXR agonist in DBA/1 wild-type and Abca1(-/-) mice. Expression of Abcg5 and Abcg8, recently implicated in biliary excretion of cholesterol and its intestinal absorption, was induced in T0901317-treated mice. Thus, activation of LXR in mice leads to enhanced hepatobiliary cholesterol secretion and fecal neutral sterol loss independent of (ABCA1-mediated) elevation of HDL and the presence of ABCA1 in liver and intestine

Essential fatty acid deficiency in mice is associated with hepatic steatosis and secretion of large VLDL particles

Essential fatty acid (EFA) deficiency in mice decreases plasma triglyceride (TG) concentrations and increases hepatic TG content. We evaluated in vivo and in vitro whether decreased hepatic secretion of TG-rich very low-density lipoprotein (VLDL) contributes to this consequence of EFA deficiency. EFA deficiency was induced in mice by feeding an EFAdeficient (EFAD) diet for 8 wk. Hepatic VLDL secretion was quantified in fasted EFAD and EFA- sufficient (EFAS) mice using the Triton WR-1339 method. In cultured hepatocytes from EFAD and EFAS mice, VLDL secretion into medium was measured by quantifying [H-3]-labeled glycerol incorporation into TG and phospholipids. Hepatic expression of genes involved in VLDL synthesis and clearance was measured, as were plasma activities of lipolytic enzymes. TG secretion rates were quantitatively similar in EFAD and EFAS mice in vivo and in primary hepatocytes from EFAD and EFAS mice in vitro. However, EFA deficiency increased the size of secreted VLDL particles, as determined by calculation of particle diameter, particle sizing by light scattering, and evaluation of the TG-to-apoB ratio. EFA deficiency did not inhibit hepatic lipase and lipoprotein lipase activities in plasma, but increased hepatic mRNA levels of apoAV and apoCII, both involved in control of lipolytic degradation of TG-rich lipoproteins. EFA deficiency does not affect hepatic TG secretion rate in mice, but increases the size of secreted VLDL particles. Present data suggest that hypotriglyceridemia during EFA deficiency is related to enhanced clearance of altered VLDL particles

A novel approach to monitor glucose metabolism using stable isotopically labelled glucose in longitudinal studies in mice

The aetiology of insulin resistance is still an enigma. Mouse models are frequently employed to study the underlying pathology. The most commonly used methods to monitor insulin resistance are the HOMA-IR, glucose or insulin tolerance tests and the hyperinsulinemic euglycaemic clamp (HIEC). Unfortunately, these tests disturb steady state glucose metabolism. Here we describe a method in which blood glucose kinetics can be determined in fasted mice without noticeably perturbing glucose homeostasis. The method involves an intraperitoneal injection of a trace amount of [6,6-H-2(2)]glucose and can be performed repeatedly in individual mice. The validity and performance of this novel method was tested in mice fed on chow or high-fat diet for a period of five weeks. After administering the mice with [6,6-2H2]glucose, decay of the glucose label was followed in small volumes of blood collected by tail tip bleeding during a 90-minute period. The total amount of blood collected was less than 120 mu L. This novel approach confirmed in detail the well-known increase in insulin resistance induced by a high-fat diet. The mice showed reduced glucose clearance rate, and reduced hepatic and peripheral insulin sensitivity. To compensate for this insulin resistance, beta-cell function was slightly increased. We conclude that this refinement of existing methods enables detailed information of glucose homeostasis in mice. Insulin resistance can be accurately determined while mechanistic insight is obtained in underlying pathology. In addition, this novel approach reduces the number of mice needed for longitudinal studies of insulin sensitivity and glucose metabolism