2H2O incorporation into hepatic acetyl-CoA and de novo lipogenesis as measured by Krebs cycle-mediated 2H-enrichment of glutamate and glutamine

In vivo deuterium magnetic resonance imaging of xenografted tumors following systemic administration of deuterated water

In vivo deuterated water (2H2O) labeling leads to deuterium (2H) incorporation into biomolecules of proliferating cells and provides the basis for its use in cell kinetics research. We hypothesized that rapidly proliferating cancer cells would become preferentially labeled with 2H and, therefore, could be visualized by deuterium magnetic resonance imaging (dMRI) following a brief period of in vivo systemic 2H2O administration. We initiated systemic 2H2O administration in two xenograft mouse models harboring either human colorectal, HT-29, or pancreatic, MiaPaCa-2, tumors and 2H2O level of ~ 8% in total body water (TBW). Three schemas of 2H2O administration were tested: (1) starting at tumor seeding and continuing for 7 days of in vivo growth with imaging on day 7, (2) starting at tumor seeding and continuing for 14 days of in vivo growth with imaging on day 14, and (3) initiation of labeling following a week of in vivo tumor growth and continuing until imaging was performed on day 14. Deuterium chemical shift imaging of the tumor bearing limb and contralateral control was performed on either day 7 of 14 after tumor seeding, as described. After 14 days of in vivo tumor growth and 7 days of systemic labeling with 2H2O, a clear deuterium contrast was demonstrated between the xenografts and normal tissue. Labeling in the second week after tumor implantation afforded the highest contrast between neoplastic and healthy tissue in both models. Systemic labeling with 2H2O can be used to create imaging contrast between tumor and healthy issue, providing a non-radioactive method for in vivo cancer imaging.

De novo lipogenesis in non-alcoholic fatty liver disease: Quantification with stable isotope tracers

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is characterized as an abnormal accumulation of triglyceride in hepatocytes. Hepatic de novo lipogenesis may play an important role in the accumulation of lipids in the liver during NAFLD. Due to the importance of lipid biosynthetic fluxes in NAFLD and T2D, tracer methodologies have been developed for their study and quantification. Here, we address novel approaches to measure and quantify DNL using stable isotope tracers. Deuterated water is a widely used tracer for quantifying DNL rates in both animal models and humans. Enrichment of lipid hydrogens from ² H2O can be resolved and quantified by ² H NMR and MS spectroscopy of isolated lipids. NMR provides a much higher level of positional enrichment information compared with MS which yields a more detailed picture of lipid biosynthetic. It can also be used to quantify low levels of lipid ¹³ C enrichment from a second tracer such as [U-¹³ C]sugar with minimal interference of one tracer with the other.

CONCLUSIONS

Despite the clear association between elevated DNL activity and increased hepatic triglyceride levels, implementation of non-destructive and novel methods to quantify DNL and its contribution to NAFLD are also of huge interest.

Effect of Duration of Exposure to Fluoride and Type of Diet on Lipid Parameters and De Novo Lipogenesis

The effect of duration of chronic treatment with fluoride (F, 50 mg/L as NaF) on the lipid profile, lipid droplets and triglycerides (TG) in liver was evaluated in mice with nonalcoholic fatty liver disease (NAFLD) previously induced by hyperlipidic diet and in animals fed normocaloric diet. In addition, the effect of F administered for a short period (20 days) was evaluated on de novo lipogenesis, by nuclear magnetic resonance. GRP78, Apo-E, and sterol regulatory element-binding protein (SREBP) were quantified by Western blotting. Our data indicate that F interferes in lipid metabolism and lipid droplets, having a different action depending on the exposure time and type of diet administered. F improved lipid parameters and reduced steatosis only when administered for a short period of time (up to 20 days) to animals fed normocaloric diet. However, when NAFLD was already installed, lipid parameters were only slightly improved at 20 days of treatment, but no effect was observed on the degree of steatosis. In addition, lipid profile was in general impaired when the animals were treated with F for 30 days, regardless of the diet. Moreover, F did not alter de novo lipogenesis in animals with installed NAFLD. Furthermore, hyperlipidic diet increased F accumulation in the body. GRP78 increased, while Apo-E and SREBP decreased in the F-treated groups. Our results provide new insights on how F affects lipid metabolism depending on the available energy source.

Glucose and Lipid Dysmetabolism in a Rat Model of Prediabetes Induced by a High-Sucrose Diet

Glucotoxicity and lipotoxicity are key features of type 2 diabetes mellitus, but their molecular nature during the early stages of the disease remains to be elucidated. We aimed to characterize glucose and lipid metabolism in insulin-target organs (liver, skeletal muscle, and white adipose tissue) in a rat model treated with a high-sucrose (HSu) diet. Two groups of 16-week-old male Wistar rats underwent a 9-week protocol: HSu diet (n = 10)-received 35% of sucrose in drinking water; Control (n = 12)-received vehicle (water). Body weight, food, and beverage consumption were monitored and glucose, insulin, and lipid profiles were measured. Serum and liver triglyceride concentrations, as well as the expression of genes and proteins involved in lipid biosynthesis were assessed. The insulin-stimulated glucose uptake and isoproterenol-stimulated lipolysis were also measured in freshly isolated adipocytes. Even in the absence of obesity, this rat model already presented the main features of prediabetes, with fasting normoglycemia but reduced glucose tolerance, postprandial hyperglycemia, compensatory hyperinsulinemia, as well as decreased insulin sensitivity (resistance) and hypertriglyceridemia. In addition, impaired hepatic function, including altered gluconeogenic and lipogenic pathways, as well as increased expression of acetyl-coenzyme A carboxylase 1 and fatty acid synthase in the liver, were observed, suggesting that liver glucose and lipid dysmetabolism may play a major role at this stage of the disease.

Metabolic evaluations of cancer metabolism by NMR-based stable isotope tracer methodologies

BACKGROUND

Cancer cells are widely recognized for being able to adapt their metabolism towards converting available nutrients into biomass to increase proliferation rates.

MATERIALS AND METHODS

We will review a series of nuclear magnetic resonance (NMR)-based stable isotope tracer methodologies for probing cancer metabolism.

RESULTS

The monitoring of such adaptations is of the utmost importance to unravel cancer metabolism and tumour growth. Several major metabolic targets have been recognized as promising foci and have been addressed by multiple studies in recent years. In this work are presented strategies to quantify glycolysis, pentose phosphate pathway, Krebs cycle turnover and de novo lipogenesis by NMR isotopomer analysis.

CONCLUSIONS

Being able to adequately define the interplay between metabolic pathways allows the monitoring of their prevalence in tissues and such information is critical for an accurate knowledge of the metabolic distinctive nature of tumours towards devising more efficient antitumorigenic strategies. Discussed methodologies are currently available in the literature, but to date, no single review has compiled all their possible uses, particularly in an interdependent perspective.

Identifying Sources of Hepatic Lipogenic Acetyl-CoA Using Stable Isotope Tracers and NMR

The role of hepatic de novo lipogenesis (DNL) in promoting fatty liver disease and hypertriglyceridemia during excessive nutrient intake is becoming firmly established. Certain nutrients such as fructose promote hepatic DNL activity and this has been at least partly attributed to their efficient conversion to the acetyl-CoA precursors of DNL. However, tracer studies indicate a paradoxically low level of fructose incorporation into lipids, which begs the question of what the actual lipogenic acetyl-CoA sources are under these and other conditions. Here, we describe novel approaches for measuring substrate contributions to lipogenic hepatic acetyl-CoA using 13C-tracers and 13C-NMR analysis of lipids and acetyl-CoA probes. We review and address aspects of hepatic intermediary fluxes and acetyl-CoA compartmentation that can confound the relationship between 13C-precursor substrate and lipogenic 13C-acetyl-CoA enrichments and demonstrate novel methodologies that can provide realistic estimates of 13C-enriched substrate contributions to DNL. The most striking realization is that the principal substrate contributors to lipogenic acetyl-CoA have yet to be identified, but they are probably not the so-called “lipogenic substrates” such as fructose. The proposed methods may improve our insight into the nutrient sources of DNL under various feeding and disease states.

Restoration of direct pathway glycogen synthesis flux in the STZ-diabetes rat model by insulin administration

Type 1 diabetes subjects are characterized by impaired direct pathway synthesis of hepatic glycogen that is unresponsive to insulin therapy. Since it is not known whether this is an irreversible defect of insulin-dependent diabetes, direct and indirect pathway glycogen fluxes were quantified in streptozotocin (STZ)-induced diabetic rats and compared with STZ rats that received subcutaneous or intraperitoneal insulin (I-SC or I-IP). Three groups of STZ rats were studied at 18 days post-STZ treatment. One group was administered I-SC and another I-IP as two daily injections of short-acting insulin at the start of each light and dark period for days 9-18. A third group did not receive any insulin, and a fourth group of nondiabetic rats was used as control. Glycogen synthesis via direct and indirect pathways, de novo lipogenesis, and gluconeogenesis were determined over the nocturnal feeding period using deuterated water. Direct pathway was residual in STZ rats, and glucokinase activity was also reduced significantly from control levels. Insulin administration restored both net glycogen synthesis via the direct pathway and glucokinase activity to nondiabetic control levels and improved the lipogenic pathway despite an inefficient normalization of the gluconeogenic pathway. We conclude that the reduced direct pathway flux is not an irreversible defect of insulin-dependent diabetes.