J.C. Fruchart

Opposite regulation of human versus mouse apolipoprotein A-I by fibrates in human apolipoprotein A-I transgenic mice.

The regulation of liver apolipoprotein (apo) A-I gene expression by fibrates was studied in human apo A-I transgenic mice containing a human genomic DNA fragment driving apo A-I expression in liver. Treatment with fenofibrate (0.5% wt/wt) for 7 d increased plasma human apo A-I levels up to 750% and HDL-cholesterol levels up to 200% with a shift to larger particles. The increase in human apo A-I plasma levels was time and dose dependent and was already evident after 3 d at the highest dose (0.5% wt/wt) of fenofibrate. In contrast, plasma mouse apo A-I concentration was decreased after fenofibrate in nontransgenic mice. The increase in plasma human apo A-I levels after fenofibrate treatment was associated with a 97% increase in hepatic human apo A-I mRNA, whereas mouse apo A-I mRNA levels decreased to 51%. In nontransgenic mice, a similar down-regulation of hepatic apo A-I mRNA levels was observed. Nuclear run-on experiments demonstrated that the increase in human apo A-I and the decrease in mouse apo A-I gene expression after fenofibrate occurred at the transcriptional level. Since part of the effects of fibrates are mediated through the nuclear receptor PPAR (peroxisome proliferator-activated receptor), the expression of the acyl CoA oxidase (ACO) gene was measured as a control of PPAR activation. Both in transgenic and nontransgenic mice, fenofibrate induced ACO mRNA levels up to sixfold. When transgenic mice were treated with gemfibrozil (0.5% wt/wt) plasma human apo A-I and HDL-cholesterol levels increased 32 and 73%, respectively, above control levels. The weaker effect of this compound on human apo A-I and HDL-cholesterol levels correlated with a less pronounced impact on ACO mRNA levels (a threefold increase) suggesting that the level of induction of human apo A-I gene is related to the PPAR activating potency of the fibrate used. Treatment of human primary hepatocytes with fenofibric acid (500 microM) provoked an 83 and 50% increase in apo A-I secretion and mRNA levels, respectively, supporting that a direct action of fibrates on liver human apo A-I production leads to the observed increase in plasma apo A4 and HDL-cholesterol.

Apolipoprotein A5, a Crucial Determinant of Plasma Triglyceride Levels, Is Highly Responsive to Peroxisome Proliferator-activated Receptor α Activators

The recently discovered APOA5 gene has been shown in humans and mice to be important in determining plasma triglyceride levels, a major cardiovascular disease risk factor. apoAV represents the first described apolipoprotein where overexpression lowers triglyceride levels. Since fibrates represent a commonly used therapy for lowering plasma triglycerides in humans, we investigated their ability to modulate APOA5 gene expression and consequently influence plasma triglyceride levels. Human primary hepatocytes treated with Wy 14,643 or fenofibrate displayed a strong induction of APOA5 mRNA. Deletion and mutagenesis analyses of the proximal APOA5 promoter firmly demonstrate the presence of a functional peroxisome proliferator-activated receptor response element. These findings demonstrate that APOA5 is a highly responsive peroxisome proliferator-activated receptor α target gene and support its role as a major mediator for how fibrates reduce plasma triglycerides in humans.

Retinoids increase human apo C-III expression at the transcriptional level via the retinoid X receptor. Contribution to the hypertriglyceridemic action of retinoids.

Hypertriglyceridemia is a metabolic complication of retinoid therapy. In this study, we analyzed whether retinoids increase the expression of apo C-III, an antagonist of plasma triglyceride catabolism. In men, isotretinoin treatment (80 mg/d; 5 d) resulted in elevated plasma apo C-III, but not apo E concentrations. In human hepatoma HepG2 cells, retinoids increased apo C-III mRNA and protein production. Transient transfection experiments indicated that retinoids increase apo C-III expression at the transcriptional level. This increased apo C-III transcription is mediated by the retinoid X receptor (RXR), since LG1069 (4-[1-(5,6,7,8-tetrahydro-3,5,5,8, 8-pentamethyl-2-naphtalenyl)ethenyl]benzoic acid), a RXR-specific agonist, but not TTNPB ((E)- 4-[2-(5,6,7,8-tetrahydro-5,5,8, 8-tetramethyl-2-naphtalenyl)propenyl]benzoic acid), a retinoic acid receptor (RAR)-specific agonist, induced apo C-III mRNA in HepG2 cells and primary human hepatocytes. Mutagenesis experiments localized the retinoid responsiveness to a cis-element consisting of two imperfect AGGTCA sequences spaced by one oligonucleotide (DR-1), within the previously identified C3P footprint site. Cotransfection assays showed that RXR, but not RAR, activates apo C-III transcription through this element either as a homo- or as a heterodimer with the peroxisome proliferator-activated receptor. Thus, apo C-III is a target gene for retinoids acting via RXR. Increased apo C-III expression may contribute to the hypertriglyceridemia and atherogenic lipoprotein profile observed after retinoid therapy.

Short term response of circulating leptin to feeding and fasting in man: influence of circadian cycle.

OBJECTIVE: To investigate whether acute feeding induces changes in circulating leptin levels in humans and whether these changes vary according to nycthemeral cycle.METHODS: First experiment. Eighteen male subjects were given a fatty meal at 08.00u2005h. Blood sampling was performed for 10u2005h following this meal. Second experiment. Thirteen male subjects were given either a mixed meal or remained fasting either at night (starting at 01.00u2005h) or during the day (starting at 13.00u2005h). Blood samples were drawn every hour for a period of 8u2005h.RESULTS: First experiment. Serum leptin levels increased progressively from a mean (±s.d.) baseline of 3.8±2.2u2005ng/ml to a value of 4.5±2.7u2005ng/ml (P<0.01) 8u2005h after the fatty meal. Second experiment. During the day, serum leptin levels increased progressively from 2.65±1.7 to 3.34±2.2u2005ng/ml (P<0.001) 6u2005h after the test-meal and decreased from 2.68±1.5 to 1.9±1.1u2005ng/ml (P<0.001) 8u2005h after the beginning of the fasting experiment. Similar results were obtained at night. No statistically significant differences in leptin levels were observed between day and night sessions in response to feeding (mean area under the curve: 3.0±4.1 vs 4.1±4.1u2005ng/ml) and fasting (−2.9±2.2 vs −1.5±2.2u2005ng/ml).CONCLUSION: In two independent experiments, human serum leptin levels increase following food intake. This response is not influenced by nycthemeral cycle.

Régulation transcriptionnelle du métabolisme du cholestérol.

Sterols and cholesterol in particular are lipids that have been studied exhaustively in view of their vital role in diverse cellular functions. Recently, it has been recognized that cholesterol is not only an essential component for the formation of membranes and the synthesis of steroid hormones and bile acids, but also is a key molecule in caveolae formation and embryonic development. Intracellular cholesterol can be derived exogenously from the plasma via receptor-mediated uptake or endogenously via de novo biosynthesis. However, as a consequence of the toxic effects of excess cholesterol in the cell and the well established contribution of hypercholesteremia to atherosclerotic diseases, it is of most importance to control cholesterol homeostasis as efficiently as possible. Until now, the regulation of the content of intracellular cholesterol was thought to be predominantly achieved by a feedback mechanism controlling a variety of genes involved in cholesterol biosynthesis and cholesterol uptake. The key factor in this process is a basic helix loop helix - leucine zipper protein, designated ADD-1/SREBP, a transcription factor activated by a cholesterol - dependent proteolytic system. Only recently, a new group of nuclear hormone receptors has been discovered that control the activation of diverse pathways involved in the utilisation of cholesterol. These receptors, including LXR, SF-1, FXR, and the recently identified receptors PXR and SXR, are activated by cholesterol intermediates or derivatives. The identification of sterol-activated nuclear receptors opens up a whole new area in the lipid and endocrinology research field. A better comprehension of the mechanism of action and the physiological role of these receptors will undoubtedly contribute to find new therapeutic ways in the treatment of metabolic disorders linked to an abberated cholesterol metabolism