Effect of norepinephrine on gluconeogenesis in perfused livers of cold-exposed rats

NAD+ /sirtuin metabolism is enhanced in response to cold-induced changes in lipid metabolism in mouse liver

The nicotinamide adenine dinucleotide (NAD⁺ )/Sirtuin (SIRT) system is linked to metabolic adaptation. This study aimed to determine the temporal profile of metabolic responses of the liver to cold exposure and changes in the hepatic NAD⁺ /SIRT system. Eight-week-old male C57BL/6 mice were individually housed in conventional cages under cold exposure (4 °C) for up to 5 days. Cold exposure decreased the hepatic triglyceride level and cholesterol level in mice by 1.7- and 1.6-fold, respectively. Lipogenic gene expression was persistently reduced, while gluconeogenic gene expression was transiently increased. Hepatic NAD⁺ /SIRT metabolism was induced during the 'cold remodeling' phase (days 1-3) and correlated with decreasing lipogenic and increasing gluconeogenic gene expression, contributing to the maintenance of whole-body lipid and glucose homeostasis.

Comparative proteomic study of liver lipid droplets and mitochondria in mice housed at different temperatures

Laboratory mice are standardly housed at around 23 °C, setting them under chronic cold stress. Metabolic changes in the liver in mice housed at thermoneutral, standard and cold temperatures remain unknown. In the present study, we isolated lipid droplets and mitochondria from their livers in a comparative proteomic study aiming to investigate the changes. According to proteomic analysis, mitochondrial tricarboxylic acid cycle (TCA cycle) and retinol metabolism are enhanced, whereas oxidative phosphorylation is not affected obviously under cold conditions, suggesting that liver mitochondria may increase TCA cycle capacity in biosynthetic pathways, as well as retinol metabolism, to help the liver to adapt. Based on proteomic and immunoblotting results, perilipin 5 and major urinary proteins are increased significantly, whereas mitochondrial pyruvate carrier is decreased dramatically under cold conditions, indicating their involvement in liver adaptation.

Cold acclimation reduces hepatic protein Kinase B and AMP-activated protein kinase phosphorylation and increases gluconeogenesis in Rats

This study investigated the molecular and metabolic responses of the liver to cold-induced thermogenesis. To accomplish that, male Wistar rats were exposed to cold (4°C) for 7 days. Livers were then extracted and used for the determination of glucose and fatty acid oxidation, glycogen content, the expression and content of proteins involved in insulin signaling, as well as in the regulation of gluconeogenesis and de novo lipid synthesis. Despite being hyperphagic, cold-acclimated rats displayed normoglycemia with reduced insulinemia, which suggests improved whole-body insulin sensitivity. However, liver protein kinase B (AKT) and glycogen synthase kinase 3 (GSK3) phosphorylations were markedly reduced along with the expressions of the insulin receptor (IR) and its substrates IRS1 and IRS2, whereas glycogen synthase (GS) phosphorylation increased. Thus, major signaling steps of the glycogen synthesis pathway in the liver were inhibited. Furthermore, glucagonemia and hepatic glucose and fatty acid oxidation were increased, whereas liver glycogen content was reduced by cold acclimation. This was accompanied by significantly elevated expressions of the gluconeogenic transcription regulators CRTC2, PGC-1α, and FoxO1, as well as of major gluconeogenic enzymes (G6Pase, FBP1, and PEPCK). Conversely, phosphorylation and contents of AMP-activated protein kinase (AMPK) and acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS) content were markedly downregulated in livers of cold-acclimated rats. In conclusion, cold acclimation suppressed hepatic glycogen synthesis and promoted profound metabolic changes in the liver so the organ could sustain its ability to regulate whole-body glucose and lipid metabolism under conditions of high-energy demand in thermogenic tissues.

A compendium and functional characterization of mammalian genes involved in adaptation to Arctic or Antarctic environments

Background

Many mammals are well adapted to surviving in extremely cold environments. These species have likely accumulated genetic changes that help them efficiently cope with low temperatures. It is not known whether the same genes related to cold adaptation in one species would be under selection in another species. The aims of this study therefore were: to create a compendium of mammalian genes related to adaptations to a low temperature environment; to identify genes related to cold tolerance that have been subjected to independent positive selection in several species; to determine promising candidate genes/pathways/organs for further empirical research on cold adaptation in mammals.

Results

After a search for publications containing keywords: “whole genome”, “transcriptome or exome sequencing data”, and “genome-wide genotyping array data” authors looked for information related to genetic signatures ascribable to positive selection in Arctic or Antarctic mammalian species. Publications related to Human, Arctic fox, Yakut horse, Mammoth, Polar bear, and Minke whale were chosen. The compendium of genes that potentially underwent positive selection in >1 of these six species consisted of 416 genes. Twelve of them showed traces of positive selection in three species. Gene ontology term enrichment analysis of 416 genes from the compendium has revealed 13 terms relevant to the scope of this study. We found that enriched terms were relevant to three major groups: terms associated with collagen proteins and the extracellular matrix; terms associated with the anatomy and physiology of cilium; terms associated with docking. We further revealed that genes from compendium were over-represented in the lists of genes expressed in the lung and liver.

Conclusions

A compendium combining mammalian genes involved in adaptation to cold environment was designed, based on the intersection of positively selected genes from six Arctic and Antarctic species. The compendium contained 416 genes that have been positively selected in at least two species. However, we did not reveal any positively selected genes that would be related to cold adaptation in all species from our list. But, our work points to several strong candidate genes involved in mechanisms and biochemical pathways related to cold adaptation response in different species.

Electronic supplementary material

The online version of this article (10.1186/s12863-017-0580-9) contains supplementary material, which is available to authorized users.

Mechanisms by which liver-specific PEPCK knockout mice preserve euglycemia during starvation

Liver-specific PEPCK knockout mice, which are viable despite markedly abnormal lipid metabolism, exhibit mild hyperglycemia in response to fasting. We used isotopic tracer methods, biochemical measurements, and nuclear magnetic resonance spectroscopy to show that in mice lacking hepatic PEPCK, 1) whole-body glucose turnover is only slightly decreased; 2) whole-body gluconeogenesis from phosphoenolpyruvate, but not from glycerol, is moderately decreased; 3) tricarboxylic acid cycle activity is globally increased, even though pyruvate cycling and anaplerosis are decreased; 4) the liver is unable to synthesize glucose from lactate/pyruvate and produces only a minimal amount of glucose; and 5) glycogen synthesis in both the liver and muscle is impaired. Thus, although mice without hepatic PEPCK have markedly impaired hepatic gluconeogenesis, they are able to maintain a near-normal blood glucose concentration while fasting by increasing extrahepatic gluconeogenesis coupled with diminishing whole-body glucose utilization.

Different preparations of zymosan induce glycogenolysis independently in the perfused rat liver. Involvement of mannose receptors, peptide-leukotrienes and prostaglandins

Zymosan (non-boiled) induced glycogenolysis biphasically, with no lag time, in the perfused rat liver. After the zymosan was boiled, it could be separated into two fractions, both of which stimulated glycogenolysis independently. The soluble fraction of boiled zymosan (zymosan sup) showed homologous desensitization, indicating that zymosan sup-induced glycogenolysis is a receptor-mediated event. Mannan (polymannose), which is known to be a biologically active component of zymosan, induced a glycogenolytic response similar to that produced by zymosan sup, and desensitized the response to the latter. Preinfusion of platelet-activating factor (PAF, 20 nM) or isoprenaline (10 microM) did not extinguish the glycogenolytic response to zymosan sup, while the response to a secondary infusion of PAF was blocked. The glycogenolytic response to zymosan sup was completely inhibited by nordihydroguaiaretic acid (NDGA, 10 microM), a lipoxygenase inhibitor, and by ONO-1078 (100 ng/ml), a leukotriene (LT) D4 receptor antagonist. On the other hand, the glycogenolytic effect of zymosan pellet (the particulate fraction of boiled zymosan) was not affected by preinfusion of zymosan sup, and was inhibited by ibuprofen (20 microM), a cyclo-oxygenase inhibitor. Prostaglandins (PGs) detected in the perfusate were augmented with infusion of zymosan pellet. Opsonization of the zymosan pellet by serum (complement) enhanced the glycogenolytic response without a lag period, and with a concomitant enhancement of PG output. Correlations between glucose production and PGs were r = 0.832 (PGD2), r = 0.872 (PGF2 alpha), r = 0.752 (PGE2) and r = 0.349 (6-oxo-PGF1 alpha). The glycogenolytic response to non-boiled zymosan was delayed and the biphasic glycogenolytic response was not observed when mannan was infused first. NDGA mimicked the effects of the preinfusion of mannan, while ibuprofen had no effect on the non-boiled-zymosan-induced glycogenolysis. These results suggest: (1) that non-boiled zymosan stimulates glycogenolysis through a mannose receptor-dependent, but unidentified, pathway, (2) that zymosan sup induces glycogenolysis via mannose receptor activation through the production of peptide-LTs but not PAF, and (3) that zymosan pellet causes glycogenolysis through the production of prostanoids, which is enhanced in the presence of complement.