Research conducted at the University of Campinas (UNICAMP) in São Paulo State, Brazil, shows that vigorous physical exercise such as strength and weight training can reduce accumulated liver fat and improve blood sugar control in obese and diabetic individuals in a short time span, even before significant weight loss occurs.
In experiments with mice, researchers at UNICAMP’s Molecular Biology of Exercise Laboratory (LaBMEx) found that two weeks of such exercise was sufficient to modify gene expression in liver tissue in ways that “burned” more stored lipids and contributed to the treatment of nonalcoholic fatty liver disease.
Cellular insulin signaling in tissue improved, and hepatic synthesis of glucose decreased.
The results of the study, which was supported by São Paulo Research Foundation—FAPESP, are published in the Journal of Endocrinology.
“Everyone knows physical exercise helps control disease.
Our research focuses on how and why this is so, on the mechanisms involved.
If we can discover a key protein whose levels rise or fall with training, we’ll have taken a step toward the development of drugs that mimic some of the benefits of physical exercise,” said Leandro Pereira de Moura, a professor at UNICAMP’s School of Applied Sciences and the principal investigator of the study.
Moura explained that excess fat in the liver causes local inflammation, which makes liver cells less sensitive to the action of insulin.
This condition can progress to cirrhosis and eventually to liver failure.
“In obese individuals at cardiometabolic risk, reducing liver fat is vital to help control diabetes,” Moura told.
“The liver should produce glucose only under fasting conditions, but if insulin signaling in tissue is impaired, the liver releases glucose into the blood stream even after ingestion of carbohydrate, when insulin levels are high, and this raises the level of blood sugar.”
Strength training for mice
To investigate the effect of strength training on the liver, an experiment involving three groups of mice was performed.
The control group, which was fed a standard diet (4 percent fat), remained lean and sedentary.
The second and third groups were fed a hyperlipidemic diet (35 percent fat) for 14 weeks, long enough for the animals to become obese and diabetic.
The second group remained sedentary, while the third group was submitted to a moderate strength training exercise protocol for 15 days after becoming obese and diabetic.
The exercise protocol consisted of climbing a staircase with a weight attached to the tail. Each day, the mice climbed the stairs 20 times at 90-second intervals. According to Moura, the protocol was designed to mimic strength training in humans.
“Before we began the experiment, we conducted tests to determine the maximum load each animal could bear.
We used a weight corresponding to 70 percent of this limit in the exercise sessions.
Our group had previously shown overtraining can contribute significantly to the development of nonalcoholic fatty liver disease.
Excessively strenuous exercise can do more harm than good,” Moura said.
The researchers opted for a short exercise protocol of only 15 days to demonstrate that the benefits observed were directly linked to strength training and not to the secondary effects of weight loss.
In fact, the researchers found that although the mice submitted to exercise training were still obese at the end of the 15-day period, their fasting blood sugar levels were normal, whereas the mice in the sedentary obese group remained diabetic until the end of the experiment.
An analysis of liver tissue showed a 25%-30% reduction in local fat in the group that performed the exercise protocol compared with the fat level of the sedentary obese group.
The quantity of proinflammatory proteins was also reduced in the exercise group, and yet the mice in that group still had approximately 150% more liver fat than the control group.
In fasting conditions, the liver is the main organ responsible for maintaining adequate blood sugar levels.
In the context of diabetes, control of gluconeogenesis (endogenous glucose production) is absent as a result of insulin resistance, and the individual can become hyperglycemic.
To evaluate the effect of strength exercise on the control of hepatic gluconeogenesis, the researchers tested the animals for tolerance of pyruvate, the main substrate used by the liver to produce glucose.
“The test consisted basically of administering pyruvate to the mice and measuring the amount of glucose produced by the liver,” Moura explained.
“We found that the trained mice produced less glucose than the sedentary obese mice even though they received the same amount of substrate.
This showed that the trained animal’s liver underwent metabolic alterations that made it more sensitive to insulin.”
Next, the researchers investigated the mechanism by which exercise reduced liver fat.
To do this, the researchers analyzed the tissue expression of genes associated with lipogenesis (synthesis of fatty acids and triglycerides, contributing to the accumulation of fat) and lipolysis (breakdown of lipids for use as an energy source by the organism).
“We compared the sedentary obese mice with the exercised mice by means of gene and protein analyses to evaluate the synthesis and oxidation of liver fat,” Moura said. “We observed a tendency towards more liver fat accumulation in the sedentary mice.”
He added that an important contribution of the study was its demonstration that strength exercise promoted beneficial alterations in tissue that was not directly acted upon by skeletomuscular contractions.
“Our next step will be to find out how this communication between muscles and liver is processed. Our hypothesis is that a protein called clusterin may be involved,” he said.
If the rise in clusterin levels induced by physical exercise is shown to be beneficial, Moura said, treatments with synthetic alternatives could be tested.
Physical exercise improves cardiomyocyte metabolism
Physical exercise increases energy metabolism
Disorders of myocardial glucose and lipid metabolism lead to changes in pathways related with myocardial energy metabolism.
Abnormalities that produce cardiac structure and function are called “metabolic remodeling of the heart,” which ultimately leads to the development of cardiomyopathy.
Glucose transporter-4 (GLUT-4) is an intracellular protein that can be translocated to cell membrane induced by insulin, and then it can participate in glucose uptake and utilization.
The expression of GLUT-4 was decreased and abnormally distributed in diabetic state, resulting in a significant decrease in glucose transport and impaired myocardium energy utilization (58).
Studies indicate that moderate exercise can upregulate GLUT-4 expression, and also can increase glucose transport and activate pyruvate dehydrogenase complexes, even in the absence of insulin (59).
It suggests that exercise can compensate for impaired energy metabolism in insulin-deficient state, which may be related to the increase of insulin-sensitive adenosine monophosphate activated protein kinase (AMPK) expression, thereby protecting pancreatic β cells. Exercise may also enhance insulin-mediated glucose transport by increasing the expression of protein kinase C-δ (60, 61).
Exercise can also increase insulin and its downstream protein expressions in the myocardium of diet-induced obesity rats, as well as forkhead box protein o1 (Foxo1) and other key regulators of pancreatic β cells, and also activate insulin signaling pathway (40). Thus, exercise can protect pancreatic β cells, promote insulin secretion, activate insulin signaling pathway, increase GLUT4 expression, improve intracellular energy metabolism, and ultimately protect cardiomyocytes.
Physical exercise enhances calcium regulation
Calcium is a crucial mediator of cell signaling in skeletal muscles for cellular functions and specific functions, including contraction, fiber-type differentiation, and energy production. Intracellular Ca2+ dyshomeostasis is one of the main markers of DCM, which can affect myocardial contractile function, directly leading to the occurrence and development of DCM.
It is even worse in altered sarcoplasmic reticulum Ca2+uptake rate accompanied by decreased function of sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) (62).
In T2DM patients, the Na+-Ca2+ exchange of cardiomyocytes is inhibited, while the sarcoplasmic reticulum Ca2+ pump is normal, and Ca2+ is gradually concentrated in the sarcoplasmic reticulum.
Thus, the amplitude and attenuation rate of Ca2+ concentration in the myocardium is decreased.
Conversely, exercise can improve the expression and activity of SERCA2a, which can regulate Ca2+ release and recapture in the myocardium. Exercise can increase Ca2+-calmodulin-dependent protein kinase phosphorylation, reduce Ca2+ efflux, facilitate Ca2+ regulation, and ultimately improves myocardial contraction and diastolic function (41).
Stølen et al. found that high intensity intermittent exercise improved myocardial contractility by restoring L-type Ca2+ channels, increasing the density of T-transverse tubules, and increasing the synchrony of Ca2+ release and excitatory contraction coupling (42).
Physical exercise improves mitochondrial function
Mitochondrion is the center of energy metabolism, and recent evidence suggests that mitochondrial dysfunction may play a critical role in the pathogenesis of DCM.
The imbalance of energy supply and demand directly leads to the decline of myocardial function and induction of DCM (63).
The ultrastructure of mitochondria in DCM shows reduced density, mitochondrial swelling, and destruction of the intima and adventitia, and an increase in mitochondrial matrix, while exercise attenuates diabetes-induced ultrastructural changes in rat cardiac tissue (43). Moderate exercise intervention has a protective effect on mitochondrial function. Exercise can regulate the key regulator of mitochondrial metabolism, peroxisome proliferator-activated receptor gamma co-stimulatory factor-1α (PGC-1α), and activate its downstream transcription factors.
Thus, it can enhance mitochondrial DNA replication and transcription, and increase mitochondria biosynthesis (44).
Furthermore, the mechanisms by which exercise improves mitochondrial function may be related to the regulation of Ca2+ in mitochondria. Ca2+ is a key metabolic enzyme activator in mitochondria, and mitochondrial Ca2+ circulatory balance can be easily affected by intracellular Ca2+ homeostasis (41, 42).
Resistance exercise improves cardiac function and mitochondrial efficiency in hearts, of diabetic rat, which were accompanied by higher expressions of mitochondrial biogenesis proteins such as PGC-1α and mitochondrial transcription factor A (TFAM) (45).
However, Veeranki et al. showed that moderate intensity exercise prevented DCM associated contractile dysfunction through restoration of mitochondrial function and connexin 43 levels in db/db mice (30).
These indicate that myocardial mitochondrial biosynthesis may be associated with exercise intensity, and exercise intensity should be further investigated about its effects on DCM.
Physical exercise relieves oxidative stress damage
Oxidative stress is considered to be a key link in the development of DCM.
Under physiological conditions, there is a balance system of oxygen free radicals and free radicals in the body.
Oxygen atoms play an important role in the redox signaling pathway.
Moderate oxidation can increase protein activity, but excessive reactive oxygen species can cause pathological changes through interaction with lipids, proteins, and DNA (64).
Hyperglycemia can directly promote the production of oxygen free radicals, induce oxidative stress, and promote cardiomyocyte apoptosis.
The mechanisms by which exercise ameliorates oxidative stress is complex, including: (1) reducing the production of reactive oxygen species.
Exercise can ameliorate the damage caused by excessive oxidative stress in the diabetic myocardium and pancreas, thereby improving glucose metabolism and reducing damage caused by reactive oxygen species (48).
Long-term exercise can also directly reduce the level of reactive oxygen species in the body by reducing the activity of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase in diabetic rats (49). (2)
Enhancing the ability of anti-oxidative stress. Exercise can increase the expression of nitric oxide synthase and nitric oxide, and ultimately enhance the antioxidant function in endothelial cells (49). Nuclear factor E2-related factor 2 (Nrf2) can regulate the expressions of antioxidants mediated by antioxidant response elements.
Studies have shown that acute exercise can promote the function of Nrf2, activate downstream antioxidant response elements, and ultimately enhance the activity of anti-oxidative stress.
In addition, knocking out the Nrf2 gene can increase the sensitivity of cardiomyocytes to oxidative stress, leading to increased oxidative damage in cells (50). Kanter et al. showed that low intensity exercise decreased the elevated tissue malondialdehyde (MDA) levels and increased the reduced activities of the enzymatic antioxidants superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and catalase (CAT) in cardiac tissue (51). It indicates that exercise improves the biological mechanisms of DCM by affecting the levels of plasminogen activator inhibitor 1 (PAI-1) and endothelial nitric oxide synthase (eNOS), and it is dependent on the intensity of exercise (52).
Physical exercise improves myocardial fibrosis
Myocardial fibrosis is the most prominent histopathological change in DCM, characterized by myocardial cell collagen deposition, interstitial fibrosis, and perivascular fibrosis, and ultimately induce the reconstruction of cardiac structure and function (53).
Numerous studies indicated that moderate exercise can decrease blood glucose, reduce myocardial fibrosis, promote myocardial reverse remodeling in diabetic rats, and improve cardiac function (30, 53).
The mechanisms may be that exercise reduces pressure overload by improving blood pressure, thereby alleviating myocardial fibrosis (53).
Exercise can increase the content of matrix metalloproteinase-2 (Mmp-2) in obese rats, increase the degradation of collagen and inhibit the formation of myocardial fibrosis (47).
The interaction of collagen with glucose can further cause chemical modification of glycated collagen to form advanced glycation end products (AGEs) that promote arterial and cardiac cirrhosis, as well as endothelial dysfunction (66).
Other mechanism by which exercise improves myocardial fibrosis may be related to improving energy metabolism, decreasing blood glucose, and myocardial glycogen deposition (54).
Novoa et al. showed that high intensity chronic exercise had a positive impact on cardiac remodeling, evidenced as reduction in myocyte hypertrophy, reduced collagen deposition, and amelioration of myocardial fibrosis (55).
Physical exercise inhibits cardiomyocyte apoptosis
Diabetes-induced cardiomyocyte apoptosis is a typical feature of DCM. Hyperglycemia can directly promote cytochrome C release to the cytoplasm by activating cytochrome C in mitochondria, triggering cascade activation of caspase-3, leading to endogenous apoptosis of cardiomyocytes.
This change plays an important role in the development of diabetic cardiac hypertrophy, myocardial remodeling, and heart failure. C-Jun N-terminal kinase is a member of the mitogen-activated protein kinase (MAPK) family, which can activate caspase-8 and the apoptotic protein Bax, and release cytochromes to promote apoptosis (67).
Veeranki et al. found that exercise can also reduce cytochrome C leakage into cytoplasm by increasing mitochondrial transmembrane potential, thus prevent cardiomyocyte apoptosis (30).
A number of studies have shown that exercise can reduce the phosphorylation of c-Jun N-terminal kinase in obese rats, block the transmission of downstream apoptotic signals.
Exercise can also increase the expression of B-cell lymphokine 2 in the myocardium of diabetic mice, which can affect the activation of pro-apoptotic proteins by binding to pro-apoptotic proteins, and ultimately play an anti-apoptotic role on cardiomyocytes in diabetic mice (56). Kanter et al. showed that low intensity exercise had a therapeutic effect on diabetes-induced morphological, biochemical, and apoptotic changes in the cardiac tissue of rats (51).
Khakdan et al. found that high intensity interval training effectively increased the expression of Sirtuin 1 (Sirt1) and B cell leukemia/lymphoma 2 (BCL-2) in diabetic rats, with improved left ventricular ejection fraction (LVEF%) and fractional shortening (FS%) (57). A recent study suggested that exercise appeared to ameliorate DCM by inhibiting endoplasmic reticulum stress-induced apoptosis in diabetic rats, which was in an intensity-dependent manner (52).
More information: Rodrigo Martins Pereira et al, Short-term strength training reduces gluconeogenesis and NAFLD in obese mice, Journal of Endocrinology (2019). DOI: 10.1530/JOE-18-0567
Journal information: Journal of Endocrinology
Provided by FAPESP