Liver glyconeogenesis: a pathway to cope with postprandial amino acid excess in high-protein fed rats?

Protein feeding promotes redistribution of endogenous glucose production to the kidney and potentiates its suppression by insulin

The aim of this study was to assess in rats the effect of protein feeding on the: 1) distribution of endogenous glucose production (EGP) among gluconeogenic organs, and 2) repercussion on the insulin sensitivity of glucose metabolism. We used gene expression analyses, a combination of glucose tracer dilution and arteriovenous balance to quantify specific organ release, and hyperinsulinemic euglycemic clamps to assess EGP and glucose uptake. Protein feeding promoted a dramatic induction of the main regulatory gluconeogenic genes (glucose-6 phosphatase and phosphoenolpyruvate carboxykinase) in the kidney, but not in the liver. As a consequence, the kidney glucose release was markedly increased, compared with rats fed a normal starch diet. Protein feeding ameliorated the suppression of EGP by insulin and the sparing of glycogen storage in the liver but had no effect on glucose uptake. Combined with the previously reported induction of gluconeogenesis in the small intestine, the present work strongly suggests that a redistribution of glucose production among gluconeogenic organs might occur upon protein feeding. This phenomenon is in keeping with the improvement of insulin sensitivity of EGP, most likely involving the hepatic site. These data shed a new light on the improvement of glucose tolerance, previously observed upon increasing the amount of protein in the diet, in type 2 diabetic patients.

Protein, amino acids and the control of food intake

PURPOSE OF REVIEW

The present review presents recent findings on peripheral and central pathways involved in protein and amino acid-induced satiety.

RECENT FINDINGS

A high-protein load leads to a higher decrease of energy intake at the next meal than carbohydrate and fat. A protein-enriched diet induces satiety, improves body composition and results in weight loss. At the peripheral level, proteins seem to induce the release of anorexigenic gut hormones cholecystokinin, glucagon-like peptide-1 and peptide YY, whereas the involvement of ghrelin remains uncertain. Energy expenditure and glucose are probably involved as metabolic signals in protein-induced satiety. Moreover, there is some evidence that the circulating level of leucine could impact food intake. Leucine has been shown to modulate the activity of the energy and nutrient sensor pathways controlled by AMPK and mTOR in the hypothalamus. Moreover, high-protein diets lead to activation of the noradrenergic/adrenergic neuronal pathway in the nucleus of the solitary tract and in melanocortin neurons in the arcuate nucleus.

SUMMARY

Complex and redundant pathways are involved in protein and amino acid-induced satiety. Significant advances have recently allowed a better understanding of the involved cellular and molecular mechanisms. The involvement of some specific area of the brain including the hypothalamus and the nucleus of the solitary tract has to be further analyzed.

The transcriptomic signature of fasting murine liver

Background

The contribution of individual organs to the whole-body adaptive response to fasting has not been established. Hence, gene-expression profiling, pathway, network and gene-set enrichment analysis and immunohistochemistry were carried out on mouse liver after 0, 12, 24 and 72 hours of fasting.

Results

Liver wet weight had declined ~44, ~5, ~11 and ~10% per day after 12, 24, 48 and 72 hours of fasting, respectively. Liver structure and metabolic zonation were preserved. Supervised hierarchical clustering showed separation between the fed, 12–24 h-fasted and 72 h-fasted conditions. Expression profiling and pathway analysis revealed that genes involved in amino-acid, lipid, carbohydrate and energy metabolism responded most significantly to fasting, that the response peaked at 24 hours, and had largely abated by 72 hours. The strong induction of the urea cycle, in combination with increased expression of enzymes of the tricarboxylic-acid cycle and oxidative phosphorylation, indicated a strong stimulation of amino-acid oxidation peaking at 24 hours. At this time point, fatty-acid oxidation and ketone-body formation were also induced. The induction of genes involved in the unfolded-protein response underscored the cell stress due to enhanced energy metabolism. The continuous high expression of enzymes of the urea cycle, malate-aspartate shuttle, and the gluconeogenic enzyme Pepck and the re-appearance of glycogen in the pericentral hepatocytes indicate that amino-acid oxidation yields to glucose and glycogen synthesis during prolonged fasting.

Conclusion

The changes in liver gene expression during fasting indicate that, in the mouse, energy production predominates during early fasting and that glucose production and glycogen synthesis become predominant during prolonged fasting.

Nonalcoholic fatty liver disease: the potential role of nutritional management

PURPOSE OF REVIEW

To review available data pertaining to dietary imbalances and metabolic alterations leading to the development of fatty liver disease and nutritional managements.

RECENT FINDINGS

The importance of treating fatty liver disease is now firmly recognized not only because of the risk of progression toward a more aggressive liver disease but also because the fatty liver is an important provider of cardiovascular risks. The ideal diet for nonalcoholic fatty liver disease/nonalcoholic steatohepatitis patients should reduce fat mass and inflammation in the adipose tissue, restore insulin sensitivity, and provide low amounts of substrates for de-novo lipogenesis, but scientific evidence to recommend specific diets is currently lacking. Moderate weight loss, low-calorie diets, reduction in saturated fatty acids intake, together with an increase in monosaturated and n-3 polyunsaturated fatty acids appear to be beneficial. Excessive consumption of high glycemic index carbohydrates appears deleterious, as it favors hyperglycemia and hyperinsulinemia and stimulates de-novo lipogenesis. Physical exercise is an important component of the approach, as it improves insulin sensitivity. Behavioral therapy promotes long-term compliance to lifestyle modifications.

SUMMARY

One panacea is unlikely to be found, the more useful approach is probably multimodal and includes tailored lifestyle modifications. Randomized controlled trials are needed to establish dietary recommendations. While awaiting such trials, reduced consumption of simple sugar, especially sweetened beverages, and incremental increase in physical activity must be encouraged.

Intestinal gluconeogenesis is a key factor for early metabolic changes after gastric bypass but not after gastric lap-band in mice

Unlike the adjustable gastric banding procedure (AGB), Roux-en-Y gastric bypass surgery (RYGBP) in humans has an intriguing effect: a rapid and substantial control of type 2 diabetes mellitus (T2DM). We performed gastric lap-band (GLB) and entero-gastro anastomosis (EGA) procedures in C57Bl6 mice that were fed a high-fat diet. The EGA procedure specifically reduced food intake and increased insulin sensitivity as measured by endogenous glucose production. Intestinal gluconeogenesis increased after the EGA procedure, but not after gastric banding. All EGA effects were abolished in GLUT-2 knockout mice and in mice with portal vein denervation. We thus provide mechanistic evidence that the beneficial effects of the EGA procedure on food intake and glucose homeostasis involve intestinal gluconeogenesis and its detection via a GLUT-2 and hepatoportal sensor pathway.

Dexamethasone sensitizes the neonatal intestine to fructose induction of intestinal fructose transporter (Slc2A5) function

The recent dramatic increase in fructose consumption is tightly correlated with an equally dramatic surge in the incidence of type 2 diabetes and obesity in children, but little is known about dietary fructose metabolism and absorption in neonates. The expression of the rat intestinal fructose transporter GLUT5 [Slc2A5, a member of the glucose transporter family (GLUT)] can be specifically induced by its substrate fructose, but only after weaning begins at 14 d of age. In suckling rats younger than 14 d old, dietary fructose cannot enhance GLUT5 expression. The aim of this study was to identify the mechanisms allowing fructose to stimulate GLUT5 during weaning. After intestines were perfused with fructose or glucose (control), using microarray hybridization we showed that of 5K genes analyzed in 10-d-old pups, only 13 were fructose responsive. Previous work found approximately 50 fructose-responsive genes in 20-d-old pups. To identify fructose-responsive genes whose expression also changed with age, intestines of 10- and 20-d-old littermate pups perfused with fructose were compared by microarray. Intestines of 10- and 20-d-old pups perfused with glucose were used to segregate age- but not fructose-responsive genes. About 28 genes were up- and 22 down-regulated in 20- relative to 10-d-old pups, under conditions of fructose perfusion, and many were found, by cluster analysis, to be regulated by corticosterone. When dexamethasone was injected into suckling pups before fructose perfusion, the expression of GLUT5 but not that of the sodium glucose cotransporter (SGLT) 1 and of GLUT2, as well as the uptake of fructose but not of glucose increased dramatically. Thus, dexamethasone, which allows dietary fructose to precociously stimulate intestinal fructose absorption, can mimic the effect of age and modify developmental timing mechanisms regulating GLUT5.