A Model of Chronic Nutrient Infusion in the Rat

Chronic exposure to excessive levels of nutrients is postulated to  affect the function of several organs and tissues and to contribute to the  ...

CRCHUM

University of Montreal

Lou/C obesity-resistant rat constitutes an original model to understand the phenomena of overweight and obesity. The aim of the present study was to identify metabolic causes for the outstanding leanness of Lou/C rat. To this end, the metabolic profiles (food intake, energy expenditure, and physical activity) and the cellular characteristics of white adipose tissue (lipogenesis, lipolysis, cellularity, and lipid composition) in 30-wk-old Lou/C rats were compared with age-matched Wistar rats. Lou/C rats exhibited a lower body weight (-45%), reduced adiposity (-80%), increased locomotor activity (+95%), and higher energy expenditure (+11%) than Wistar rats. Epididymal adipose tissue of Lou/C rat was twice lower than that of Wistar rat due to both a reduction in both adipocyte size (-25%) and number (three times). Basal lipolysis and sensitivity to noradrenaline were similar; however, the responsiveness to noradrenaline was lower in adipocytes from Lou/C compared with that from Wistar rats. Lipidomic analysis of plasma, adipose tissue, and liver revealed profound differences in lipid composition between the two strains. Of note, the desaturation indexes (ratio C16:1/C16:0 and C18:1/C18:0) were lower in Lou/C, indicating a blunted activity of delta-9-desaturase such as stearoyl-coenzyme A-desaturase-1. Increased physical activity, increased energy expenditure, and white adipose tissue cellularity are in good agreement with previous observations suggesting that a higher sympathetic tone in Lou/C could contribute to its lifelong leanness.

Lou/C obesity-resistant rat exhibits hyperactivity, hypermetabolism, alterations in white adipose tissue cellularity, and lipid tissue profiles.

The concept of glucolipotoxicity refers to the combined, deleterious effects of elevated glucose and fatty acid levels on pancreatic beta-cell function and survival. Significant progress has been made in recent years towards a better understanding of the cellular and molecular basis of glucolipotoxicity in the beta cell. The permissive effect of elevated glucose on the detrimental actions of fatty acids stems from the influence of glucose on intracellular fatty acid metabolism, promoting the synthesis of cellular lipids. The combination of excessive levels of fatty acids and glucose therefore leads to decreased insulin secretion, impaired insulin gene expression, and beta-cell death by apoptosis, all of which probably have distinct underlying mechanisms. Recent studies from our laboratory have identified several pathways implicated in fatty acid inhibition of insulin gene expression, including the extracellular-regulated kinase (ERK1/2) pathway, the metabolic sensor Per-Arnt-Sim kinase (PASK), and the ATF6 branch of the unfolded protein response. We have also confirmed in vivo in rats that the decrease in insulin gene expression is an early defect which precedes any detectable abnormality in insulin secretion. While the role of glucolipotoxicity in humans is still debated, the inhibitory effects of chronically elevated fatty acid levels has been clearly demonstrated in several studies, at least in individuals genetically predisposed to developing type 2 diabetes. It is therefore likely that glucolipotoxicity contributes to beta-cell failure in type 2 diabetes as well as to the decline in beta-cell function observed after the onset of the disease.

Glucolipotoxicity of the pancreatic beta cell.

C57Bl/6 mice develop obesity and mild hyperglycemia when fed a high-fat diet (HFD). Although diet-induced obesity (DIO) is a widely studied model of type 2 diabetes, little is known about beta-cell failure in these mice.

Beta-cell failure in diet-induced obese mice stratified according to body weight gain: secretory dysfunction and altered islet lipid metabolism without steatosis or reduced beta-cell mass.

Chronic kidney disease (CKD) is frequently associated with malnutrition, anorexia, and hyperleptinemia. This study was designed to test the hypothesis that a component of the uremic milieu may trigger leptin release by adipocytes. To this end, mouse 3T3-L1 adipocytes were incubated for 16 hours in culture medium containing urea (80 mM) or plasma from either healthy volunteers or patients with CKD (20%, v/v). Uremic plasma and not urea induced a large release of leptin (+557%, P < .01). These results suggest that the hyperleptinemia reported in patients with CKD, could be, at least in part, because of an overproduction of leptin by the adipose tissue.

Human uremic plasma and not urea induces exuberant secretion of leptin in 3T3-L1 adipocytes.

Cellular homeostasis requires intrinsic sensing mechanisms to temper function in the face of prolonged activity. In the pancreatic Î²-cell, glucose is likely a physiological trigger that activates an adaptive response to stimulation, thereby maintaining cellular homeostasis. Immediate early genes (IEGs) are activated as a first line of defense in cellular homeostasis and are largely responsible for transmitting an environmental cue to a cellular response. Here we examine the regulation and function of the novel Î²-cell IEG, neuronal PAS domain protein 4 (Npas4). Using MIN6 cells, mouse and human islets, as well as in vivo infusions, we demonstrate that Npas4 is expressed within pancreatic islets and is upregulated by Î²-cell depolarizing agents. Npas4 tempers Î²-cell function through a direct inhibitory interaction with the insulin promoter and by blocking the potentiating effects of GLP-1 without significantly reducing glucose-stimulated secretion. Finally, Npas4 expression is induced by classical endoplasmic reticulum (ER) stressors and can prevent thapsigargin- and palmitate-induced dysfunction and cell death. These results suggest that Npas4 is a key activity-dependent regulator that improves Î²-cell efficiency in the face of stress. We posit that Npas4 could be a novel therapeutic target in type 2 diabetes that could both reduce ER stress and cell death and maintain basal cell function.

Npas4 Is a Novel Activity-Regulated Cytoprotective Factor in Pancreatic Î²-Cells.

In pancreatic Î² cells, glucose induces the binding of the transcription factor pancreatic duodenal homeobox-1 (PDX-1) to the insulin gene promoter to activate insulin gene transcription. At low glucose levels, glycogen synthase kinase 3 beta (GSK3Î²) is known to phosphorylate PDX-1 on C-terminal serine residues, which triggers PDX-1 proteasomal degradation. We previously showed that the serine/threonine Per-ARNT-Sim domain-containing kinase (PASK) regulates insulin gene transcription via PDX-1. However, the mechanisms underlying this regulation are unknown. In this study, we aimed to identify the role of PASK in the regulation of PDX-1 phosphorylation, protein expression and stability in insulin secreting cells and isolated rat islets of Langerhans. We observed that glucose induces a decrease in overall PDX-1 serine phosphorylation, and that overexpression of wild-type PASK mimics this effect. In vitro, PASK directly phosphorylates GSK3Î² on its inactivating phosphorylation site Ser9. Overexpression of a kinase-dead (KD), dominant-negative version of PASK blocks glucose-induced Ser9 phosphorylation of GSK3Î². Accordingly, GSK3Î² Ser9 phosphorylation is reduced in islets from pask-null mice. Overexpression of wild-type (WT) PASK or KD GSK3Î² protects PDX-1 from degradation and results in increased PDX-1 protein abundance. Conversely, overexpression of KD PASK blocks glucose-induction of PDX-1 protein. We conclude that PASK phosphorylates and inactivates GSK3Î², thereby preventing PDX-1 serine phosphorylation and alleviating GSK3Î²-mediated PDX-1 protein degradation in pancreatic Î² cells.

Per-Arnt-Sim Kinase Regulates Pancreatic Duodenal Homeobox-1 Protein Stability via Phosphorylation of Glycogen Synthase Kinase 3 Beta in Pancreatic Beta Cells.

Bader Zarrouki

Montreal Diabetes Research Center

CRCHUM

A Model of Chronic Nutrient Infusion in the Rat

Bader Zarrouki

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CRCHUM

A Model of Chronic Nutrient Infusion in the Rat

Montreal Diabetes Research Center