Jean-Francois Decaux

Long-chain fatty acids regulate liver carnitine palmitoyltransferase I gene (L-CPT I) expression through a peroxisome-proliferator-activated receptor alpha (PPARalpha)-independent pathway.

Liver carnitine palmitoyltransferase I (L-CPT I) catalyses the transfer of long-chain fatty acid (LCFA) for translocation across the mitochondrial membrane. Expression of the L-CPT I gene is induced by LCFAs as well as by lipid-lowering compounds such as clofibrate. Previous studies have suggested that the peroxisome-proliferator-activated receptor alpha (PPARalpha) is a common mediator of the transcriptional effects of LCFA and clofibrate. We found that free LCFAs rather than acyl-CoA esters are the signal metabolites responsible for the stimulation of L-CPT I gene expression. Using primary culture of hepatocytes we found that LCFAs failed to stimulate L-CPT I gene expression both in wild-type and PPARalpha-null mice. These results suggest that the PPARalpha-knockout mouse does not represent a suitable model for the regulation of L-CPT I gene expression by LCFAs in the liver. Finally, we determined that clofibrate stimulates L-CPT I through a classical direct repeat 1 (DR1) motif in the promoter of the L-CPT I gene while LCFAs induce L-CPT I via elements in the first intron of the gene. Our results demonstrate that LCFAs can regulate gene expression through PPARalpha-independent pathways and suggest that the regulation of gene expression by dietary lipids is more complex than previously proposed.

Hormonal regulation of liver phosphoenolpyruvate carboxykinase and glucokinase gene expression at weaning in the rat

During the suckling period, the rats are fed continuously with milk, which is a high-fat low-carbohydrate diet (HF). At weaning, milk is progressively replaced by the rats laboratory chow which is a high-carbohydrate low-fat diet (HCHO), and this is accompanied by large hormonal modifications: an increase in plasma insulin and a decrease in plasma glucagon concentrations, and by marked changes in metabolic pathways in liver: decrease in hepatic gluconeogenesis and increase in glycolysis and lipogenesis. Most of the data concerning these changes are related to maximal activity of enzymes. The recent availability of specific cDNA probes for phosphoenolpyruvate carboxykinase (PEPCK), and glucokinase (GK) has allowed the study of the role of pancreatic hormones and nutrition in the changes of the expression of these genes at weaning in the rat. Regarding phosphoenolpyruvate carboxykinase gene transcription, the concentration of mRNA as well as the activity of PEPCK are elevated in the liver of suckling rat until the onset of weaning, 21 d after delivery. After weaning to a HCHO diet, both mRNA and activity of PEPCK rapidly decrease to a very low level. In contrast, weaning on an HF diet, which maintains high plasma glucagon and low plasma insulin levels, does not decrease in plasma glucagon concentration and a 90% decrease in PEPCK gene transcription and PEPCK mRNA concentration in 1 h. Regarding glucokinase gene transcription, the concentration of mRNA as well as the activity of GK are not detectable before 15 d after birth in the liver of the rat. They markedly increase when the newborn are weaned on an HCHO diet but not when they are weaned on an HF diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of triiodothyronine and retinoic acid on glucokinase gene expression in neonatal rat hepatocytes

Glucokinase (EC 2.7.1.2) first appears in rat liver two weeks after birth and increases rapidly after weaning on to a high-carbohydrate diet. We investigated the role of triiodothyronine and retinoic acid in the absence of insulin on the first expression of the glucokinase gene in primary cultures of hepatocytes from 10 day-old rats. These two hormones were able to induce a rapid accumulation of liver glucokinase mRNA, secondarily to a stimulation of gene transcription during the first 24 h of culture. Moreover, the effects of individual hormones were not additive. Finally, glucokinase mRNA stability was not modified by these hormones. This suggests that triiodothyronine and retinoic acid act on glucokinase gene at the transcriptional.

Effect of weaning on different diets on hepatic gluconeogenesis in the rat.

The capacity of gluconeogenesis from lactate (10 mmol/l) was studied on isolated hepatocytes in 15-day-old suckling and in 28-day-old rats weaned at the age of 19 days on various diets. Weaning on a high-fat carbohydrate-free diet allowed to maintain a high gluconeogenic rate. By contrast, as soon as the carbohydrate content of the weaning diet was sufficient to meet the glucose needs of the newborn, the gluconeogenic capacity was reduced. The amount of fat in the weaning diet had no influence on the gluconeogenic capacity. Changes in the hepatic gluconeogenic rate at weaning were inversely correlated with the plasma insulin/glucagon molar ratio.

Intramitochondrial factors controlling hepatic fatty acid oxidation at weaning in the rat

Fatty acid oxidation was studied in isolated liver mitochondria of rats during the suckling‐weaning transition. The oxidation rate of oleyl‐CoA and palmitoylcarnitine was reduced 2.5‐fold in rats weaned on a high‐carbohydrate diet compared to suckling rats, when acetyl‐CoA produced by β‐oxidation was directed towards ketone‐body synthesis. Weaning on a high‐fat diet minimized this change. Channeling of acetyl‐CoA towards citrate synthesis doubled the oxidation rate of both substrates in HC‐weaned rats. Thus, in addition to changes in carnitine palmitoyltransferase I activity, the β‐hydroxymethylglutaryl‐CoA synthase pathway is also involved in the decreased fatty acid oxidation at weaning. This was confirmed by measurement of β‐hydroxymethylglutaryl‐CoA synthase pathway activity.

Evidence that hepatic mitochondrial mass decreases during the first sixteen hours following birth in starved newborn rats

Fatty acid oxidation increases in newborn rats between 0 and 16 h after birth. We have tested the hypothesis that such a rise might be due to an increase in hepatic mitochondrial mass. The ratio total activity/specific activity for cytochrome c oxidase and citrate synthase was used as an index that reflected the changes in mitochondrial mass. This ratio was decreased by about 25% 16 h after delivery, indicating that hepatic mitochondrial mass was lower 16 h after birth than at birth in starved rats. We conclude that changes in the mitochondrial mass are not responsible for the increase in liver capacity to oxidize fatty acids.