We hypothesized that the lipid-activated transcription factor, the peroxisome proliferator-activated receptor α (PPARα), plays a pivotal role in the cellular metabolic response to fasting. Short-term starvation caused hepatic steatosis, myocardial lipid accumulation, and hypoglycemia, with an inadequate ketogenic response in adult mice lacking PPARα (PPARα−/−), a phenotype that bears remarkable similarity to that of humans with genetic defects in mitochondrial fatty acid oxidation enzymes. In PPARα+/+ mice, fasting induced the hepatic and cardiac expression of PPARα target genes encoding key mitochondrial (medium-chain acyl-CoA dehydrogenase, carnitine palmitoyltransferase I) and extramitochondrial (acyl-CoA oxidase, cytochrome P450 4A3) enzymes. In striking contrast, the hepatic and cardiac expression of most PPARα target genes was not induced by fasting in PPARα−/− mice. These results define a critical role for PPARα in a transcriptional regulatory response to fasting and identify the PPARα−/− mouse as a potentially useful murine model of inborn and acquired abnormalities of human fatty acid utilization.

https://www.pnas.org/content/pnas/96/13/7473.full.pdf

A critical role for the peroxisome proliferator-activated receptor alpha (PPARalpha) in the cellular fasting response: the PPARalpha-null mouse as a model of fatty acid oxidation disorders.

Multihormonal regulation of phosphoenolpyruvate carboxykinase gene transcription. The dominant role of insulin.

Defect in Peroxisome Proliferator-activated Receptor α-inducible Fatty Acid Oxidation Determines the Severity of Hepatic Steatosis in Response to Fasting *

Effect of fatty acids on energy coupling processes in mitochondria

Type 2 diabetes is caused by a complex set of interactions between genetic and environmental factors Recent work has shown that human type 2 diabetes is a constellation of disorders associated with polymorphisms in a wide array of genes, with each individual gene accounting for <1% of disease risk (1) Moreover, type 2 diabetes involves dysfunction of multiple organ systems, including impaired insulin action in muscle and adipose, defective control of hepatic glucose production, and insulin deficiency caused by loss of β-cell mass and function (2) This complexity presents challenges for a full understanding of the molecular pathways that contribute to the development of this major disease Progress in this area may be aided by the recent advent of technologies for comprehensive metabolic analysis, sometimes termed “metabolomics” Herein, we summarize key metabolomics methodologies, including nuclear magnetic resonance (NMR) and mass spectrometry (MS)-based metabolic profiling technologies, and discuss “nontargeted” versus “targeted” approaches Examples of the application of these tools to diabetes and metabolic disease research at the cellular, animal model, and human disease levels are summarized, with a particular focus on insights gained from the more quantitative targeted methodologies We also provide early examples of integrated analysis of genomic, transcriptomic, and metabolomic datasets for gaining knowledge about metabolic regulatory networks and diabetes mechanisms and conclude by discussing prospects for future insights

In principal, metabolomics can provide certain advantages relative to other “omics” technologies (genomics, transcriptomics, proteomics) in diabetes research: 1 ) Estimates vary, but one current source, the Human Metabolome Database (HMDB)-Canada (3), currently lists ∼6,500 discrete small molecule metabolites, significantly less than the estimate of 25,000 genes, 100,000 transcripts, and 1,000,000 proteins 2 ) Metabolomics measures chemical phenotypes that are the net result of genomic, transcriptomic, and proteomic variability, therefore providing the most integrated profile of biological status 3 ) Metabolomics is in …

https://diabetes.diabetesjournals.org/content/diabetes/58/11/2429.full.pdf

Metabolomics applied to diabetes research: moving from information to knowledge

Coordinate control of intermediary metabolism in rat liver by the insulin/glucagon ratio during starvation and after glucose refeeding. Regulatory significance of long-chain acyl-CoA and cyclic AMP.

The effect of acute cold exposure on the concentration of glucagon in the blood was investigated in man and in intact and adrenalectomized rats. In man fasted overnight acute cold exposure, which caused a twofold increase in O2-consumption resulted in a rapid rise in plasma glucagon. The levels of insulin and blood glucose remained unaltered, while the concentration of serum free fatty acids and β-hydroxybutyrate increased. In fasted intact rats acute cold exposure lead to similar effects. A close parallelism between the rise in plasma glucagon and the concentration of hepatic cycloAMP was observed. Adrenalectomy did not impair the cold induced rise in plasma glucagon and hepatic cycloAMP. It is concluded that acute cold exposure caused a rapid rise in the concentration of plasma glucagon leading to an increase in the concentration of hepatic cycloAMP, thus enhancing the rate of hepatic gluconeogenesis and ketogenesis. As these alterations were similar in the absence of glucocorticoids and medulla-derived catecholamines, it is suggested that glucagon may play a role in the metabolic adaptation to acute cold exposure.

Rapid rise in plasma glucagon induced by acute cold exposure in man and rat.

Long-term perfusion of the isolated rat liver maintenance of its functional state by use of a fluorocarbon emulsion

The concentration of insulin and glucagon in peripheral blood and the concentration of cAMP in liver was followed in rats throughout a 48 hour starvation period and up to 6 hours after refeeding glucose or casein. By so changing the insulin/glucagon molar ratio from minimum to maximum values, simultaneous inverse changes in the concentration of hepatic cAMP could be induced. The study, thus, suggests that during a starvation-refeeding cycle the level of cAMP in the liver is regulated predominantly by the insulin/glucagon ratio in the blood. Possible criticisms of this conclusion are discussed.

Concentration of Cyclic AMP in Rat Liver as a Function of the Insulin/Glucagon Ratio in Blood under Standardized Physiological Conditions11

The physiologic significance of glucocorticoids and insulin in the regulation of hepatic gluconeogenesis was investigated during a 48-hr starvation period by studying the factors presumed to control the rate of glucose synthesis in the final gluconeogenetic pathway. Rats were used, in which glucocorticoids were removed by adrenalectomy before starvation, and in which serum insulin was kept constant before and after food withdrawal by prefeeding with a proteinfree diet. It was found that adrenalectomized rats at constantly low serum insulin levels responded to starvation as rapidly, and to the same degree, as intact control subjects (1) by a significant increase in plasma glucagon and, consequently, in hepatic cAMP concentration; (2) by a coordinate elevation of the activities of hepatic pyruvate carboxylase, P-enolpyruvate carboxykinase, and fructose-1,6-diphosphatase; (3) by systematic alterations in the concentration of effectors of gluconeogenetic key enzymes; (4) by a shifting of the cytoplasmic NAD system towards the reduced state; (5) by a decrease in the intrahepatic concentration of glycogenic precursor substrates. These results suggest that the hepatic gluconeogenic response to starvation with respect to the regulatory factors 1–5 occurs independently from changes in the concentration of plasma glucocorticoids and insulin. The crossing over of the gluconeogenetic intermediates between pyruvate and P-enolpyruvate (PEP), which was observed in intact but not in adrenalectomized rats, supports the assumption that during starvation, glucocorticoids enhance the rate of glucose production by the liver predominantly by permitting hepatic cAMP to stimulate the yet undefined mechanism, which has been demonstrated in the isolated perfused rat liver to control the substrate flow between pyruvate and PEP.

W. Tarnowski

Papers

Coordinate control of intermediary metabolism in rat liver by the insulin/glucagon ratio during starvation and after glucose refeeding. Regulatory significance of long-chain acyl-CoA and cyclic AMP.

Rapid rise in plasma glucagon induced by acute cold exposure in man and rat.

Long-term perfusion of the isolated rat liver maintenance of its functional state by use of a fluorocarbon emulsion

Concentration of Cyclic AMP in Rat Liver as a Function of the Insulin/Glucagon Ratio in Blood under Standardized Physiological Conditions11

Physiologic significance of glucocorticoids and insulin in the regulation of hepatic gluconeogenesis during starvation in rats