Abstract: Starvation elicits a complex adaptive response in an organism. No information on transcriptional regulation of metabolic adaptations is available. We, therefore, studied the gene expression profiles of brain, small intestine, kidney, liver and skeletal muscle in mice that were subjected to 0-72 hours of fasting. Functional-category enrichment, text mining and network analyses were employed to scrutinize the overall adaptation, aiming to identify responsive pathways, processes and networks, and their regulation. The observed transcriptomics response did not follow the accepted "carbohydrate-lipid-protein succession" of expenditure of energy substrates. Instead, these processes were activated simultaneously in different organs during the entire period. The most prominent changes occurred in lipid and steroid metabolism, especially in the liver and kidney. They were accompanied by suppression of the immune response and cell turnover, particularly in the small intestine, and by increased proteolysis in the muscle. The brain was extremely well protected from the sequels of starvation. 60% of the identified overconnected transcription factors were organ-specific, 6% were common for 4 organs, with nuclear receptors as protagonists - accounting for almost 40% of all transcriptional regulators during fasting. The common transcription factors were PPARα, HNF4α, GCRα, AR, SREBP1 and -2, FOXOs, EGR1, c-JUN, c-MYC, SP1, YY1, and ETS1. Our data strongly suggest that the control of metabolism in four metabolically active organs is exerted by transcription factors that are activated by nutrient signals and serves, at least partly, to prevent irreversible brain damage. Our data strongly suggest that the control of metabolism in four metabolically active organs is exerted by transcription factors that are activated by nutrient signals and serves, at least partly, to prevent irreversible brain damage.
Abstract: The main endogenous source of glutamine is de novo synthesis in striated muscle via the enzyme glutamine synthetase (GS). The mice in which GS is selectively but completely eliminated from striated muscle with the Cre-loxP strategy (GS-KO/M mice) are, nevertheless, healthy and fertile. Compared with controls, the circulating concentration and net production of glutamine across the hindquarter were not different in fed GS-KO/M mice. Only a approximately 3-fold higher escape of ammonia revealed the absence of GS in muscle. However, after 20 h of fasting, GS-KO/M mice were not able to mount the approximately 4-fold increase in glutamine production across the hindquarter that was observed in control mice. Instead, muscle ammonia production was approximately 5-fold higher than in control mice. The fasting-induced metabolic changes were transient and had returned to fed levels at 36 h of fasting. Glucose consumption and lactate and ketone-body production were similar in GS-KO/M and control mice. Challenging GS-KO/M and control mice with intravenous ammonia in stepwise increments revealed that normal muscle can detoxify approximately 2.5 micromol ammonia/g muscle.h in a muscle GS-dependent manner, with simultaneous accumulation of urea, whereas GS-KO/M mice responded with accumulation of glutamine and other amino acids but not urea. These findings demonstrate that GS in muscle is dispensable in fed mice but plays a key role in mounting the adaptive response to fasting by transiently facilitating the production of glutamine. Furthermore, muscle GS contributes to ammonia detoxification and urea synthesis. These functions are apparently not vital as long as other organs function normally.
Abstract: Starvation induces massive perturbations in metabolic pathways involved in energy metabolism, but its effect on the metabolism of lipids, particularly cholesterol, is little understood.
Abstract: Recent reports indicate that glycosphingolipids play an important role in regulation of carbohydrate metabolism. We have shown that the iminosugar N-(5'-adamantane-1'-yl-methoxy)-pentyl-1-deoxynojirimycin (AMP-DNM), an inhibitor of the enzyme glucosylceramide synthase, is a potent enhancer of insulin signaling in rodent models for insulin resistance and type 2 diabetes. In this study, we determined whether AMP-DNM also affects lipid homeostasis and, in particular, the reverse cholesterol transport pathway. Treatment of C57BL/6J mice with AMP-DNM for 5 weeks decreased plasma levels of triglycerides and cholesterol by 35%, whereas neutral sterol excretion increased twofold. Secretion of biliary lipid also increased twofold, which resulted in a similar rise in bile flow. This effect was not due to altered expression levels or kinetics of the various export pumps involved in bile formation. However, the bile salt pool size increased and the expression of Cyp7A1 was up-regulated. In vitro experiments using HepG2 hepatoma cell line revealed this to be due to inhibition of fibroblast growth factor-19 (FGF19)-mediated suppression of Cyp7A1 via the FGF receptor. Conclusion: Pharmacological modulation of glycosphingolipid metabolism showed surprising effects on lipid homeostasis in C57BL/6J mice. Upon administration of 100 mg AMP-DNM/kg body weight/day, plasma cholesterol and triglyceride levels decreased, biliary lipid secretion doubled and also the endpoint of reverse cholesterol transport, neutral sterol excretion, doubled.
Abstract: Nonalcoholic fatty liver disease (NAFLD) is associated with obesity, insulin resistance, and type 2 diabetes. The hyperinsulinemia that occurs as a consequence of insulin resistance is thought to be an important contributor to the development of fatty liver. We have shown that the iminosugar N-(5'-adamantane-1'-yl-methoxy)-pentyl-1-deoxynojirimycin (AMP-DNM), an inhibitor of the enzyme glucosylceramide synthase, is a potent enhancer of insulin signaling in rodent models for insulin resistance and type 2 diabetes. The present study was designed to assess the impact of AMP-DNM on insulin levels, liver triglyceride synthesis, and gene expression profile. Treatment of ob/ob mice with AMP-DNM restored insulin signaling in the liver, corrected blood glucose values to levels found in lean mice, and decreased insulin concentration. The expression of sterol regulatory element-binding protein 1c target genes involved in fatty acid synthesis normalized. AMP-DNM treatment significantly reduced liver to body weight ratio and reversed hepatic steatosis, comprising fat as well as inflammatory markers. In addition, AMP-DNM treatment corrected to a large extent the gene expression profile of ob/ob mice livers toward the profile of lean mice. CONCLUSION: Pharmacological lowering of glycosphingolipids with the iminosugar AMP-DNM is a promising approach to restore insulin signaling and improve glucose homeostasis as well as hepatic steatosis.
Abstract: 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.
Abstract: The gut is a major energy consumer, but a comprehensive overview of the adaptive response to fasting is lacking. Gene-expression profiling, pathway analysis, and immunohistochemistry were therefore carried out on mouse small intestine after 0, 12, 24, and 72 hours of fasting.
Abstract: Food deprivation results in metabolic, structural, and functional changes in the small intestine that influences gut mucosal integrity, epithelial cell proliferation, mucin synthesis, and other processes. The underlying mechanisms are still unclear, which lead to the study of molecular effects of short-term and long-term starvation in the intestine of mice. A comparative proteomics approach, combining two-dimensional gel electrophoresis with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, was used to identify intestinal proteins whose expression is changed under different starvation conditions (0, 12, 24, and 72 h). In total, the expression levels of 80 protein spots changed significantly between the different groups. The results demonstrate that after 12 h of starvation, mainly proteins involved in glycolysis and energy metabolism show decreased expression levels. Starvation for 24 h results in a down-regulation of proteins involved in protein synthesis and amino acid metabolism. Simultaneously, proteins with a protective role, e.g., reg I and II, glutathione peroxidase 3, and carbonic anhydrase 3, are clearly up-regulated. The last starvation phase (72 h) is characterized by increased ezrin expression, which may enhance villus morphogenesis critical for survival. Together, these results provide novel insights in the intestinal starvation response and may contribute to improved nutritional support during conditions characterized by malnutrition.
