The effects of exercise on lipogenic enzyme activity and glyceride synthesis by liver homogenates of diabetic rats

Hepatic gene expression in mouse models of non-alcoholic fatty liver disease after acute exercise

AIM

Non-alcoholic fatty liver disease (NAFLD) patients benefit from physical exercise. This study aimed to investigate the effect of acute exercise on hepatic gene expression in different mouse models of NAFLD.

METHODS

C57BL/6J mice were fed with a control (CD) or a high fat (HFD) diet. AlbCrePten^flox/flox (Pten-KO) and Fxr^-/- mice, two genetic models of NAFLD with insulin hypersensitivity and resistance, respectively, were fed with CD. After 4 weeks, mice were randomly assigned to exercise or sedentariness. Mice were killed 15 min or 3 h after the running/sedentary period. Genome-wide hepatic gene expression was evaluated with the Illumina Micro-array platform. Quantitative polymerase chain reaction confirmed changes in gene expression.

RESULTS

Acute exercise transiently affected the expression of genes involved in the immune response in C57BL/6 mice fed with CD and this effect normalized in the recovery phase. Acute exercise affected genes involved in gluconeogenesis in the insulin resistant Fxr^-/- model. Genes involved in lipid metabolism were affected in C57BL/6 mice fed with CD, but not in mouse models of NAFLD. Genes involved in DNA damage response pathways were deregulated only in C57BL/6 mice fed with CD and not in mouse models of NAFLD.

CONCLUSION

The simultaneous analysis of different NAFLD models revealed that an acute exercise bout affects hepatic gene expression differentially according to animal models and that most of the differentially expressed genes are involved in glucose and fatty acid metabolism, immune regulation, and DNA damage response.

Effects of physical training with different intensities of effort on lipid metabolism in rats submitted to the neonatal application of alloxan

BACKGROUND

Type 2 diabetes mellitus (T2DM) is a chronic disease that is characterized by insulin resistance. Its development is directly connected with the inability of insulin to exert its action, not just on carbohydrate metabolism but also on primarily on lipid metabolism. The present study aimed to compare the effects of continuous, intermittent, and strength training on serum and tissue variables on the lipid metabolism of alloxan rats.

METHODS

Wistar rats were divided into eight groups: sedentary alloxan (SA), sedentary control (SC), continuous training alloxan (CA), intermittent training alloxan (IA), strength training alloxan (StA), continuous training control (CC), intermittent training control (IC) and strength training control (StC). Alloxan (250 mg/kg bw) was injected into neonatal rats at 6 days of age. The continuous training protocol consisted of 12 weeks of swimming training for 1 uninterrupted hour/day, five days/week, supporting a load that was 5% bw. The intermittent training protocol consisted of 12 weeks of swimming training with 30 s of activity interrupted by 30 s of rest, for a total of 20 min/day, five days/week, supporting a load that was 15% bw. The strength-training protocol consisted of 12 weeks of training, five days/week with 4 sets of 10 jumps in water with 1 min rest between sets, supporting a load that was a 50% bw.

RESULTS

At 28 days, the alloxan animals exhibited higher insulin resistance as measured by the disappearance of glucose serum (% Kitt/min) during the ITT. At 120 days, the sedentary alloxan animals showed higher FFA values than continuous and intermittent training alloxan. In addition, the alloxan animals that underwent intermittent and strength training showed lower FFA values compared to the corresponding controls. The continuous training protocol was less effective than the strength training protocol for reducing the levels of total cholesterol in the alloxan animals. Serum total lipid values revealed that intermittent training increased serum levels in alloxan animals

CONCLUSION

Thus, it was concluded that physical training at different intensities of effort is of great importance in attenuation and control of changes in the lipid metabolism in alloxan animals.

Putative factors that may modulate the effect of exercise on liver fat: insights from animal studies

An increase in intrahepatic triglyceride (IHTG) content is the hallmark of nonalcoholic fatty liver disease (NAFLD) and is strongly associated with insulin resistance and dyslipidemia. Although regular aerobic exercise improves metabolic function, its role in regulating fat accumulation in the liver is incompletely understood, and human data are scarce. Results from exercise training studies in animals highlight a number of potential factors that could possibly mediate the effect of exercise on liver fat, but none of them has been formally tested in man. The effect of exercise on IHTG content strongly depends on the background diet, so that exercise is more effective in reducing IHTG under conditions that favor liver fat accretion (e.g., when animals are fed high-fat diets). Concurrent loss of body weight or visceral fat does not appear to mediate the effect of exercise on IHTG, whereas sex (males versus females), prandial status (fasted versus fed), and duration of training, as well as the time elapsed from the last bout of exercise could all be affecting the observed exercise-induced changes in IHTG content. The potential importance of these factors remains obscure, thus providing a wide array of opportunities for future research on the effects of exercise (and diet) on liver fat accumulation.

Exercise and fat accumulation in the human liver

PURPOSE OF REVIEW

Fat accumulation in the liver is strongly associated with metabolic dysfunction. Regular exercise improves many cardiometabolic risks factors; however, its effect on intrahepatic triglyceride (IHTG) content remains elusive. This article summarizes available data regarding the effects of exercise on IHTG.

RECENT FINDINGS

Several but not all observational studies report negative associations of habitual physical activity and cardiorespiratory fitness with IHTG and the prevalence of fatty liver. Aerobic exercise training in combination with hypocaloric diet reduces IHTG by a considerable amount (20-60%), even when weight loss is mild (<5%); hence weight loss per se may not be a critical factor. Longitudinal studies involving exercise training without dietary restriction and no weight loss demonstrate that increased cardiorespiratory fitness and reduced intra-abdominal adiposity are not invariably associated with liver fat depletion, whereas relatively large exercise-induced reductions in IHTG content (20-40%) can occur even in the absence of changes in body weight, body composition, or visceral adipose tissue. Although the majority of studies have examined aerobic training, resistance exercise has also been shown to be inversely associated with the prevalence of fatty liver in humans and effectively reduces IHTG content in animals.

SUMMARY

Exercise does hold promise as an effective treatment for hepatic steatosis; this field of research is still in its infancy, and there is much more to be learned.

Activation of the mitogen-activated protein kinase (MAPK) signalling pathway in the liver of mice is related to plasma glucose levels after acute exercise

AIMS/HYPOTHESIS

We aimed to identify, in the liver of mice, signal transduction pathways that show a pronounced regulation by acute exercise. We also aimed to elucidate the role of metabolic stress in this response.

METHODS

C57Bl6 mice performed a 60 min run on a treadmill under non-exhaustive conditions. Hepatic RNA and protein lysates were prepared immediately after running and used for whole-genome-expression analysis, quantitative real-time PCR and immunoblotting. A subset of mice recovered for 3 h after the treadmill run. A further group of mice performed the treadmill run after having received a vitamin C- and vitamin E-enriched diet over 4 weeks.

RESULTS

The highest number of genes differentially regulated by exercise in the liver was found in the mitogen-activated protein kinase (MAPK) signalling pathway, with a pronounced and transient upregulation of the transcription factors encoded by c-Fos (also known as Fos), c-Jun (also known as Jun), FosB (also known as Fosb) and JunB (also known as Junb) and phosphorylation of hepatic MAPK. Acute exercise also activated the p53 signalling pathway. A major role for oxidative stress is unlikely since the antioxidant-enriched diet did not prevent the activation of the MAPK pathway. In contrast, lower plasma glucose levels after running were related to enhanced levels of MAPK signalling proteins, similar to the upregulation of Igfbp1 and Pgc-1alpha (also known as Ppargc1a). In the working muscle the activation of the MAPK pathway was weak and not related to plasma glucose concentrations.

CONCLUSIONS/INTERPRETATION

Metabolic stress evidenced as low plasma glucose levels appears to be an important determinant for the activation of the MAPK signalling pathway and the transcriptional response of the liver to acute exercise.

Effects of exercise training on diet-induced lipogenic enzymes and body composition in rats

It is well known that a carbohydrate-rich diet promotes lipogenesis via induction of hepatic lipogenic enzymes, whereas chronic physical exercise reduces body fat. The purpose of this study was to investigate whether exercise training can decrease diet-induced hepatic lipogenic enzymes and total body fat composition. Female Sprague-Dawley rats were randomly divided into exercise trained (T) and sedentary (S) groups. Training was performed on a treadmill for 8 weeks for up to 25 m/minute, 15% grade for 90 minutes/day, 5 days/week. The T and S groups were further divided into four dietary treatments. Fifty percent of the calories for each diet were from a basal ingredient and the other 50% were from either cornstarch (C), glucose (G), fructose (Fr), or fat (Fa). T significantly decreased body fat and increased body water content in all dietary groups (p < 0.01), but had no effect on body weight. Fr rats had significantly heavier liver weight than did the remaining groups. Activities of hepatic fatty acid synthase, ATP-citrate lyase (CL), malic enzyme (ME), pyruvate kinase, and glucose-6-phosphate dehydrogenase were significantly higher in Fr and lower in Fa compared with C and G. Training significantly increased activities of CL and ME in Fa, but had little effect on these enzymes in other groups. These data suggest that the fat-reducing effect of conventional exercise training is elicited by regulatory mechanisms other than by a reduction of hepatic lipogenic enzymes.

Lipogenic potential of liver from morbidly obese patients with and without non-insulin-dependent diabetes

Intra-abdominal liver biopsies were obtained during surgery from fasted obese patients with non-insulin-dependent diabetes mellitus (NIDDM), obese normoglycemic controls, and lean controls. Lipid synthesis was studied in freshly isolated hepatocytes and liver homogenates from the three groups of subjects. Incorporation of 3H2O into the lipids of hepatocytes was determined in the absence and presence of insulin (0.1 mumol/L). The activities of five enzymes involved in fatty acid synthesis, and the incorporation of 14C-glycerol-3-phosphate into lipids were determined in liver homogenates. Basal lipid synthesis by hepatocytes was not different in the three groups of patients. Insulin stimulated lipogenesis by 8% +/- 30% in the lean controls, 33% +/- 8% in the obese controls and 17% +/- 6% in the NIDDM patients. No significant differences in the activities of the five enzymes that are involved in de novo fatty acid synthesis among the three groups of patients were observed. Similarly, incorporation of 14C-glycerol-3-phosphate by liver homogenates, in the presence of saturating or submaximal concentrations of fatty acids, did not differ among the three groups. These results show that under the experimental conditions of this study, including the fasted state of the patients, the basal capacity of liver of NIDDM patients to synthesize fatty acids or glycerides is the same as that of liver from obese and lean controls. Thus, it is likely that an increase in fatty acid flux into a liver with normal lipogenic potential may contribute to the increased synthesis of triglycerides by the liver of these patients in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)