Octanoate oxidation measured by 13C-NMR spectroscopy in rat skeletal muscle, heart, and liver

Challenges of Spatially Resolved Metabolism in Cancer Research

Stable isotope-resolved metabolomics comprises a critical set of technologies that can be applied to a wide variety of systems, from isolated cells to whole organisms, to define metabolic pathway usage and responses to perturbations such as drugs or mutations, as well as providing the basis for flux analysis. As the diversity of stable isotope-enriched compounds is very high, and with newer approaches to multiplexing, the coverage of metabolism is now very extensive. However, as the complexity of the model increases, including more kinds of interacting cell types and interorgan communication, the analytical complexity also increases. Further, as studies move further into spatially resolved biology, new technical problems have to be overcome owing to the small number of analytes present in the confines of a single cell or cell compartment. Here, we review the overall goals and solutions made possible by stable isotope tracing and their applications to models of increasing complexity. Finally, we discuss progress and outstanding difficulties in high-resolution spatially resolved tracer-based metabolic studies.

Acetyl-CoA production by specific metabolites promotes cardiac repair after myocardial infarction via histone acetylation

Myocardial infarction (MI) is accompanied by severe energy deprivation and extensive epigenetic changes. However, how energy metabolism and chromatin modifications are interlinked during MI and heart repair has been poorly explored. Here, we examined the effect of different carbon sources that are involved in the major metabolic pathways of acetyl-CoA synthesis on myocardial infarction and found that elevation of acetyl-CoA by sodium octanoate (8C) significantly improved heart function in ischemia reperfusion (I/R) rats. Mechanistically, 8C reduced I/R injury by promoting histone acetylation which in turn activated the expression of antioxidant genes and inhibited cardiomyocyte (CM) apoptosis. Furthermore, we elucidated that 8C-promoted histone acetylation and heart repair were carried out by metabolic enzyme medium-chain acyl-CoA dehydrogenase (MCAD) and histone acetyltransferase Kat2a, suggesting that 8C dramatically improves cardiac function mainly through metabolic acetyl-CoA-mediated histone acetylation. Therefore, our study uncovers an interlinked metabolic/epigenetic network comprising 8C, acetyl-CoA, MCAD, and Kat2a to combat heart injury.

Mitochondria Matter: Systemic Aspects of Nonalcoholic Fatty Liver Disease (NAFLD) and Diagnostic Assessment of Liver Function by Stable Isotope Dynamic Breath Tests

The liver plays a key role in systemic metabolic processes, which include detoxification, synthesis, storage, and export of carbohydrates, lipids, and proteins. The raising trends of obesity and metabolic disorders worldwide is often associated with the nonalcoholic fatty liver disease (NAFLD), which has become the most frequent type of chronic liver disorder with risk of progression to cirrhosis and hepatocellular carcinoma. Liver mitochondria play a key role in degrading the pathways of carbohydrates, proteins, lipids, and xenobiotics, and to provide energy for the body cells. The morphological and functional integrity of mitochondria guarantee the proper functioning of β-oxidation of free fatty acids and of the tricarboxylic acid cycle. Evaluation of the liver in clinical medicine needs to be accurate in NAFLD patients and includes history, physical exam, imaging, and laboratory assays. Evaluation of mitochondrial function in chronic liver disease and NAFLD is now possible by novel diagnostic tools. "Dynamic" liver function tests include the breath test (BT) based on the use of substrates marked with the non-radioactive, naturally occurring stable isotope 13C. Hepatocellular metabolization of the substrate will generate 13CO2, which is excreted in breath and measured by mass spectrometry or infrared spectroscopy. Breath levels of 13CO2 are biomarkers of specific metabolic processes occurring in the hepatocyte cytosol, microsomes, and mitochondria. 13C-BTs explore distinct chronic liver diseases including simple liver steatosis, non-alcoholic steatohepatitis, liver fibrosis, cirrhosis, hepatocellular carcinoma, drug, and alcohol effects. In NAFLD, 13C-BT use substrates such as α-ketoisocaproic acid, methionine, and octanoic acid to assess mitochondrial oxidation capacity which can be impaired at an early stage of disease. 13C-BTs represent an indirect, cost-effective, and easy method to evaluate dynamic liver function. Further applications are expected in clinical medicine. In this review, we discuss the involvement of liver mitochondria in the progression of NAFLD, together with the role of 13C-BT in assessing mitochondrial function and its potential use in the prevention and management of NAFLD.

Exploring Liver Mitochondrial Function by 13C-Stable Isotope Breath Tests: Implications in Clinical Biochemistry

The liver is at the crossroad of key metabolic processes, which include detoxification, glycolipidic storage and export, and protein synthesis. The gut-liver axis, moreover, provides hepatocytes with a series of bacterial products and metabolites, which contribute to maintain liver function in health and disease. Breath tests (BTs) are developed as diagnostic tools for indirect, rapid, noninvasive assessment of several metabolic processes in the liver. BTs monitor the appearance of CO₂ in breath as a marker of a specific substrate metabolized in the liver, typically within microsomes, cytosol, or mitochondria. The noninvasiveness of BTs originates from the use of the, nonradioactive, naturally occurring stable isotope ¹³C marking a specific substrate which is metabolized in the liver, leading to the appearance of ¹³CO₂ in expired air. Some substrates (ketoisocaproic acid, methionine, and octanoic acid) provide information about dynamic liver mitochondrial function in health and disease. In humans, the application of ¹³C-breath tests ranges from nonalcoholic and alcoholic liver diseases to liver cirrhosis, hepatocarcinoma, preoperative and postoperative assessment of liver function, and drug-induced liver damage. ¹³C-BTs are an indirect, cost-effective, and easy method to evaluate dynamic liver function and gastric kinetics in health and disease, with ongoing studies focusing on further applications in clinical medicine.

Detection of myocardial medium-chain fatty acid oxidation and tricarboxylic acid cycle activity with hyperpolarized [1-13 C]octanoate

Under normal conditions, the heart mainly relies on fatty acid oxidation to meet its energy needs. Changes in myocardial fuel preference are noted in the diseased and failing heart. The magnetic resonance signal enhancement provided by spin hyperpolarization allows the metabolism of substrates labeled with carbon-13 to be followed in real time in vivo. Although the low water solubility of long-chain fatty acids abrogates their hyperpolarization by dissolution dynamic nuclear polarization, medium-chain fatty acids have sufficient solubility to be efficiently polarized and dissolved. In this study, we investigated the applicability of hyperpolarized [1-¹³ C]octanoate to measure myocardial medium-chain fatty acid metabolism in vivo. Scanning rats infused with a bolus of hyperpolarized [1-¹³ C]octanoate, the primary metabolite observed in the heart was identified as [1-¹³ C]acetylcarnitine. Additionally, [5-¹³ C]glutamate and [5-¹³ C]citrate could be respectively resolved in seven and five of 31 experiments, demonstrating the incorporation of oxidation products of octanoate into the tricarboxylic acid cycle. A variable drop in blood pressure was observed immediately following the bolus injection, and this drop correlated with a decrease in normalized acetylcarnitine signal (acetylcarnitine/octanoate). Increasing the delay before infusion moderated the decrease in blood pressure, which was attributed to the presence of residual gas bubbles in the octanoate solution. No significant difference in normalized acetylcarnitine signal was apparent between fed and 12-hour fasted rats. Compared with a solution in buffer, the longitudinal relaxation of [1-¹³ C]octanoate was accelerated ~3-fold in blood and by the addition of serum albumin. These results demonstrate the potential of hyperpolarized [1-¹³ C]octanoate to probe myocardial medium-chain fatty acid metabolism as well as some of the limitations that may accompany its use.

Priority use of medium-chain fatty acids during high-intensity exercise in cross-country skiers

Background

One of the topics discussed in sports science is the use of medium-chain saturated fat as an energy-saving nutrient additive when approaching high-intensity exercise. The purpose of this study was to compare the blood concentrations of medium-chain and long-chain fatty acids (FAs) across different intensity loads.

Methods

Fifteen male highly trained athletes from the Russian cross-country skiing team participated in the study. Blood samples were drawn at rest, at the peak of veloergometric test with a growing exercise load till exhaustion (97–100% VO2max), and after competitions. The plasma FA profile was determined using gas-liquid chromatography.

Results

We observed a substantial increase in the concentrations of capric acid (С10:0) (+ 164.1%), lauric acid (С12:0) (+ 223.9%), and myristic acid (С14:0) (+ 130.2%) in skiers after a sprint distance (1.3 km). A less intense increase in the concentrations of these acids (p < 0.05) was observed after a middle length distance or cycle exercise «until exhaustion». No significant differences in long-chain saturated FA content relative to baselines during exercise tests or competitions were revealed.

Conclusions

In conclusion, the obtained results demonstrate activation of the lipolysis and the oxidation of medium-chain FA involved in the energy supply for highly trained athletes at maximum exercise loads.

Short- and medium-chain fatty acids in energy metabolism: the cellular perspective

Short- and medium-chain fatty acids (SCFAs and MCFAs), independently of their cellular signaling functions, are important substrates of the energy metabolism and anabolic processes in mammals. SCFAs are mostly generated by colonic bacteria and are predominantly metabolized by enterocytes and liver, whereas MCFAs arise mostly from dietary triglycerides, among them milk and dairy products. A common feature of SCFAs and MCFAs is their carnitine-independent uptake and intramitochondrial activation to acyl-CoA thioesters. Contrary to long-chain fatty acids, the cellular metabolism of SCFAs and MCFAs depends to a lesser extent on fatty acid-binding proteins. SCFAs and MCFAs modulate tissue metabolism of carbohydrates and lipids, as manifested by a mostly inhibitory effect on glycolysis and stimulation of lipogenesis or gluconeogenesis. SCFAs and MCFAs exert no or only weak protonophoric and lytic activities in mitochondria and do not significantly impair the electron transport in the respiratory chain. SCFAs and MCFAs modulate mitochondrial energy production by two mechanisms: they provide reducing equivalents to the respiratory chain and partly decrease efficacy of oxidative ATP synthesis.

Evaluation of the 13C-octanoate breath test as a surrogate marker of liver damage in animal models

BACKGROUND

Octanoate (also known as sodium octanoate), a medium-chain fatty acid metabolized in the liver, is a potential substrate for non-invasive breath testing of hepatic mitochondrial beta-oxidation.

METHODS

We evaluated the 13C-octanoate breath test (OBT) for assessing injury in acute hepatitis and two rat models of liver cirrhosis, first testing octanoate absorption (per os or intraperitoneally (i.p.)) in normal rats. We then induced acute hepatitis with thioacetamide (300 mg/kg/i.p., 24-h intervals). Liver injury end points were serum aminotransferase levels and 13C-OBT (24 and 48 h following initial injection). Thioacetamide (200 mg/kg/i.p., twice per week, 12 weeks) was used to induce liver cirrhosis. OBT and liver histological assessment were performed every 4 weeks. Bile duct ligation (BDL) was used to induce cholestatic liver injury. We completed breath tests with 13C-OBT and 13C-methacetin (MBID), liver biochemistry, and liver histology in BDL and sham-operated rats (baseline, 6, 14, 20 days post-BDL).

RESULTS

Octanoate absorbs well by either route. Peak amplitudes and cumulative percentage dose recovered at 30 and 60 min (CPDR30/60), but not peak time, correlated with acute hepatitis. Fibrosis stage 3 at week 8 significantly correlated with each OBT parameter. Cholestatic liver injury (serum bilirubin, ALP, gamma-GT, liver histology) was associated with significant suppression of the maximal peak values and CPDR30/60, respectively (P<0.05),using MBID but not 13C-octanoate.

CONCLUSIONS

OBT is sensitive for potentially evaluating liver function in rat models of acute hepatitis and thioacetamide-induced liver cirrhosis but not in cholestatic liver injury. The MBID test may be better for evaluation of cholestatic liver disease in this model.

Dynamic MRS and MRI of skeletal muscle function and biomechanics

MR is a powerful technique for studying the biomechanical and functional properties of skeletal muscle in vivo in health and disease. This review focuses on 31P, 1H and 13C MR spectroscopy for assessment of the dynamics of muscle metabolism and on dynamic 1H MRI methods for non-invasive measurement of the biomechanical and functional properties of skeletal muscle. The information thus obtained ranges from the microscopic level of the metabolism of the myocyte to the macroscopic level of the contractile function of muscle complexes. The MR technology presented plays a vital role in achieving a better understanding of many basic aspects of muscle function, including the regulation of mitochondrial activity and the intricate interplay between muscle fiber organization and contractile function. In addition, these tools are increasingly being employed to establish novel diagnostic procedures as well as to monitor the effects of therapeutic and lifestyle interventions for muscle disorders that have an increasing impact in modern society.

Is the pattern of solid-phase gastric emptying different between genders?

BACKGROUND

In the 13C-octanoate breath test, the shape of the 13CO2 excretion curve in the ascending portion reflects a pattern of gastric emptying (GE). Recent scintigraphic studies have revealed an overall delay in solid GE in fertile women compared with men. However, it remains unknown whether women have a different GE pattern compared with men. As a symptomatic delay in solid GE is specific to the female gender, it could be hypothesized that the 13CO2 excretion curve is different in shape between genders.

MATERIALS AND METHODS

Because the ascending gradient of the 13CO2 excretion curve is often biphasic, the dual function of y(t) = ( a1 . tb1 + a2 . tb2) e(-K.t) was applied to fit the breath data, where a1, b1, a2, b2, and K are constants. Assessed on the 4 h-based breath samples obtained after ingestion of a 320-kcal muffin containing 100 mg 13C-octanoate, the time versus 13CO2 excretion curve was created from 31 adult volunteers (15 men and 16 women). The curve shape was characterized by the dual function, and was compared between genders.

RESULTS

In both genders, the ascending gradient exhibited the biphasic feature, characterized by an initial steep rise and the subsequent blunted increase, while the descending gradient followed the monotonous decay. The initial rise was steeper and the subsequent increase was more blunted in women than in men.

CONCLUSION

Women exhibit a gender-specific pattern of the 13CO2 excretion profile. A possible explanation for this gender difference is that the post-gastric feedback regulation is more potent in women than in men.

Total body metabolism of 13C-octanoic acid is preserved in patients with non-alcoholic steatohepatitis, but differs between women and men

OBJECTIVES

Among numerous factors which account for the pathogenesis of non-alcoholic steatohepatitis (NASH), hepatic mitochondrial beta-oxidation is considered to play a pivotal role. We performed a (13)C-based breath test with a medium-chain fatty acid to non-invasively assess total body beta-oxidation in patients with NASH and in healthy controls.

METHODS

We performed a simplified (13)CO(2)-based breath test in 16 patients with histologically proven NASH and 24 healthy controls. One hundred milligrams of sodium (13)C-octanoate dissolved in 200 ml of water were orally administered and breath samples were collected before and during 3 h following administration. The samples were analysed for the cumulative (13)CO(2) recovery (%-cum-dose) by non-dispersive infrared spectrometry. Additionally, data of 69 patients who had undergone a C-octanoate breath test for the assessment of gastric emptying were retrospectively evaluated for the %-cum-dose.

RESULTS

The cumulative (13)CO(2) recovery 3 h after the administration of the substrate did not differ among patients with NASH and controls (34.6 +/- 7.0% vs. 34.6 +/- 6.5%, P = 0.90). Compared with men, women yielded a significantly higher cumulative (13)CO(2) excretion in both controls (30.1 +/- 5.7% vs. 38.5 +/- 4.4%, P = 0.0008) and NASH patients (30.2 +/- 5.4% vs. 39.0 +/- 6.5%, P = 0.031). Forty-two of 69 patients (61%) of the gastric emptying group showed a normal gastric emptying rate. Among these patients, women also demonstrated a tendency for a higher (13)CO(2) recovery compared with men (P = 0.055). This was not the case in 27 patients with delayed gastric emptying (P = 0.47).

CONCLUSIONS

Though hepatic mitochondrial function might be impaired in patients with NASH, total beta-oxidation of octanoic acid remains normal. Gender-specific metabolic modifications seem to account for significant differences of the cumulative (13)CO(2) recovery in women and men. This may have further consequences for the appraisal of (13)C breath tests which involve octanoic acid. Further trials focusing on the assessment of body composition and energy expenditure could contribute essential further information.

Differential effects of thyroid hormones on energy metabolism of rat slow- and fast-twitch muscles

Thyroid hormone (TH) is an important regulator of mitochondrial content and activity. As mitochondrial content and properties differ depending on muscle-type, we compared mitochondrial regulation and biogenesis by T3 in slow-twitch oxidative (soleus) and fast-twitch mixed muscle (plantaris). Male Wistar rats were treated for 21 to 27 days with T3 (200 microg/kg/day). Oxidative capacity, regulation of mitochondrial respiration by substrates and phosphate acceptors, and transcription factors were studied. In soleus, T3 treatment increased maximal oxygen consumption (Vmax) and the activities of citrate synthase (CS) and cytochrome oxidase (COX) by 100%, 45%, and 71%, respectively (P < 0.001), whereas in plantaris only Vmax increased, by 39% (P < 0.01). ADP-independent respiration rate was increased in soleus muscle by 216% suggesting mitochondrial uncoupling. Mitochondrial substrate utilization in soleus was also influenced by T3, as were mitochondrial enzymes. Lactate dehydrogenase (LDH) activity was elevated in soleus and plantaris by 63% and 11%, respectively (P < 0.01), and soleus creatine kinase was increased by 48% (P < 0.001). T3 increased the mRNA content of the transcriptional co-activator of mitochondrial genes, PGC-1alpha, and the I and IV COX subunits in soleus. The muscle specific response to thyroid hormones could be explained by a lower content of TH receptors in plantaris than soleus. Moreover, TRalpha mRNA level decreased further after T3 treatment. These results demonstrate that TH has a major effect on mitochondrial content, regulation and coupling in slow oxidative muscle, but to a lesser extent in fast muscle, due to the high expression of TH receptors and PGC-1alpha transcription factor.