Insulin resistance without elevated mammalian target of rapamycin complex 1 activity in muscles of mice fed a high-fat diet

Health Effects of Alternate Day Fasting Versus Pair-Fed Caloric Restriction in Diet-Induced Obese C57Bl/6J Male Mice

Alternate day fasting (ADF) induces weight loss and improves various markers of health in rodents and humans. However, it is unclear whether the benefits of ADF are derived from the lower caloric intake of ADF or from the 24-h fasting period. Therefore, this study directly compared selected markers for health – such as glucose control, body weight, liver triglycerides, T cell frequencies, and others – in high-fat (60% calories from fat) diet-induced obese mice subjected to either ADF or caloric restriction (CR). Obese mice were randomly assigned to one of four groups: (1) ADF: remained on the high-fat diet, but fed on alternate days (n = 5), (2) PF: remained on the high-fat diet, but pair-fed to the ADF group (n = 5), (3) LF: moved to a chow ad libitum diet (n = 5; 17% calories from fat), and (4) HF: remained on the high-fat ad libitum diet (n = 5). An additional group of non-obese mice maintained on a chow diet since weaning were used as controls (CON: n = 5). After 10 weeks, ADF, PF, and LF mice ate fewer kcals, had a lower body mass, had smaller epididymal fat pads, improved glucose tolerance, and had a lower hepatic triglyceride content relative to HF mice (p < 0.05), but none reached that of CON mice in these measures. T cell frequencies of the spleen, blood, and mesenteric lymph nodes were reduced in ADF, PF, and HF compared to the CON group. Importantly, there were no significant differences between the ADF and PF groups in any of the measurements made in the current study. These data suggest that ADF, PF, and LF diets each lead to improved markers of health relative to high-fat diet-induced obese mice, and that the caloric restriction associated with ADF is the major factor for the noted improvements.

Obese mice on a high-fat alternate-day fasting regimen lose weight and improve glucose tolerance

Alternate-day fasting (ADF) causes body weight (BW) loss in humans and rodents. However, it is not clear that ADF while maintaining a high-fat (HF) diet results in weight loss and the accompanying improvement in control of circulating glucose. We tested the hypotheses that a high-fat ADF protocol in obese mice would result in (i) BW loss, (ii) improved glucose control, (iii) fluctuating phenotypes on 'fasted' days when compared to 'fed' days and (iv) induction of torpor on 'fasted days'. We evaluated the physiological effects of ADF in diet-induced obese mice for BW, heart rate (HR), body temperature (T_b ), glucose tolerance, insulin responsiveness, blood parameters (leptin, insulin, free fatty acids) and hepatic gene expression. Diet-induced obese male C57BL/6J mice lost one-third of their pre-diet BW while on an ADF diet for 10 weeks consisting of HF food. The ADF protocol improved glucose tolerance and insulin sensitivity, although mice on a fast day were less glucose tolerant than the same mice on a fed day. ADF mice on a fast day had low circulating insulin, but had an enhanced response to an insulin-assisted glucose tolerance test, suggesting the impaired glucose tolerance may be a result of insufficient insulin production. On fed days, ADF mice were the warmest, had a high HR and displayed hepatic gene expression and circulating leptin that closely mimicked that of mice fed an ad lib HF diet. ADF mice never entered torpor as assessed by HR and T_b . However, on fast days, they were the coolest, had the slowest HR, and displayed hepatic gene expression and circulating leptin that closely mimicked that of Chow-Fed mice. Collectively, the ADF regimen with a HF diet in obese mice results in weight loss, improved blood glucose control, and daily fluctuations in selected physiological and biochemical parameters in the mouse.

Manganese [III] Tetrakis [5,10,15,20]-Benzoic Acid Porphyrin Reduces Adiposity and Improves Insulin Action in Mice with Pre-Existing Obesity

The superoxide dismutase mimetic manganese [III] tetrakis [5,10,15,20]-benzoic acid porphyrin (MnTBAP) is a potent antioxidant compound that has been shown to limit weight gain during short-term high fat feeding without preventing insulin resistance. However, whether MnTBAP has therapeutic potential to treat pre-existing obesity and insulin resistance remains unknown. To investigate this, mice were treated with MnTBAP or vehicle during the last five weeks of a 24-week high fat diet (HFD) regimen. MnTBAP treatment significantly decreased body weight and reduced white adipose tissue (WAT) mass in mice fed a HFD and a low fat diet (LFD). The reduction in adiposity was associated with decreased caloric intake without significantly altering energy expenditure, indicating that MnTBAP decreases adiposity in part by modulating energy balance. MnTBAP treatment also improved insulin action in HFD-fed mice, a physiologic response that was associated with increased protein kinase B (PKB) phosphorylation and expression in muscle and WAT. Since MnTBAP is a metalloporphyrin molecule, we hypothesized that its ability to promote weight loss and improve insulin sensitivity was regulated by heme oxygenase-1 (HO-1), in a similar fashion as cobalt protoporphyrins. Despite MnTBAP treatment increasing HO-1 expression, administration of the potent HO-1 inhibitor tin mesoporphyrin (SnMP) did not block the ability of MnTBAP to alter caloric intake, adiposity, or insulin action, suggesting that MnTBAP influences these metabolic processes independent of HO-1. These data demonstrate that MnTBAP can ameliorate pre-existing obesity and improve insulin action by reducing caloric intake and increasing PKB phosphorylation and expression.

Effects of a High Fat Diet and Voluntary Wheel Running Exercise on Cidea and Cidec Expression in Liver and Adipose Tissue of Mice

Cidea and Cidec play an important role in regulating triglyceride storage in liver and adipose tissue. It is not known if the Cidea and Cidec genes respond to a high fat diet (HFD) or exercise training, two interventions that alter lipid storage. The purpose of the present study was to determine the effect of a HFD and voluntary wheel running (WR) on Cidea and Cidec mRNA and protein expression in adipose tissue and liver of mice. A HFD promoted a significant increase in Cidea and Cidec mRNA levels in adipose tissue and liver. The increase in Cidea and Cidec mRNAs in adipose tissue and liver in response to a HFD was prevented by WR. Similar to the changes in Cidea mRNA, Cidea protein levels in adipose tissue significantly increased in response to a HFD, a process that was, again, prevented by WR. However, in adipose tissue the changes in Cidec mRNA did not correspond to the changes in Cidec protein levels, as a HFD decreased Cidec protein abundance. Interestingly, in adipose tissue Cidea protein expression was significantly related to body weight (R=.725), epididymal adipose tissue (EWAT) mass (R=.475) and insulin resistance (R=.706), whereas Cidec protein expression was inversely related to body weight (R=-.787), EWAT mass (R=-.706), and insulin resistance (R=-.679). Similar to adipose tissue, Cidea protein expression in liver was significantly related to body weight (R=.660), EWAT mass (R=.468), and insulin resistance (R=.599); however, unlike adipose tissue, Cidec protein levels in liver were not related to body weight or EWAT mass and only moderately associated with insulin resistance (R=-.422, P=0.051). Overall, our findings indicate that Cidea is highly associated with adiposity and insulin resistance, whereas Cidec is related to insulin sensitivity. The present study suggests that Cide proteins might play an important functional role in the development of obesity, hepatic steatosis, as well as the pathogenesis of type 2 diabetes.

Disassociation of insulin action and Akt/FOXO signaling in skeletal muscle of older Akt-deficient mice

The purpose of the present study was to determine the effect of Akt gene ablation on Akt/Forkhead Box O (FOXO) signaling and atrogene expression. This was accomplished by studying wild-type (WT) and isoform-specific Akt knockout (Akt1(-/-) and Akt2(-/-)) mice. The ability of insulin to promote Akt phosphorylation on Ser(473) was significantly lower in extensor digitorum longus (EDL) and soleus muscles from Akt1(-/-) and Akt2(-/-) mice compared with WT mice. Total Akt1 protein levels were significantly lower in EDL muscles of Akt2(-/-) mice compared with WT mice, a process that appears to be posttranscriptionally regulated as Akt1 mRNA levels were unchanged. The loss of Akt1 protein in EDL muscles of Akt2(-/-) mice does not appear to be due to insulin resistance because 4 mo of a high-fat diet failed to reduce Akt1 protein levels in muscles of WT mice. Although FOXO3a phosphorylation and atrogin-1 expression were unaltered in muscles of Akt1(-/-) and Akt2(-/-) mice, the expression of the atrogenes Bnip3 and gabarapl were significantly elevated in muscles of both Akt1 and Akt2 knockout mice. Finally, the expression of striated activator of Rho signaling was significantly increased in muscles of Akt2(-/-) mice compared with Akt1(-/-) and WT mice. Our results demonstrate that the ablation of Akt isoforms disassociates insulin action and Akt/FOXO signaling to atrogenes.

Leucine improves glucose and lipid status in offspring from obese dams, dependent on diet type, but not caloric intake

Previously, we showed that offspring from obese rat dams were hyperphagic, with increased adiposity, hyperlipidaemia and glucose intolerance associated with increased orexigenic neuropeptide expression after fasting. Mammalian target of rapamycin (mTOR) can inhibit food intake through a hypothalamic action. As we previously showed that maternal obesity down-regulated hypothalamic mTOR, in the present study, we hypothesised that dietary leucine supplementation would activate hypothalamic mTOR to reduce food intake, thus limiting metabolic disorders in offspring from obese dams, regardless of postweaning diet. Obesity was induced in Sprague-Dawley females by high-fat diet (HFD) for 5 weeks before mating, throughout gestation and lactation. Male pups from HFD-fed mothers were weaned onto chow or HFD; within each dietary group, half were supplied with leucine via drinking water (1.5%) versus water control for 10 weeks. Those from chow-fed mothers were fed chow and water. Maternal obesity led to increased adiposity in chow-fed offspring. Postweaning HFD consumption exaggerated adiposity, hyperglycaemia, hyperinsulinaemia and hyperlipidaemia. Supplementation with leucine doubled leucine intake and increased hypothalamic mTOR activation; however, appetite regulation was not affected. A reduction in blood lipid levels was observed in offspring regardless of diet, as well as improved glucose tolerance in HFD-fed rats. In HFD-fed rats, up-regulated carnitine palmitoyl-transferase-1 and peroxisome-proliferator-activated receptor-γ coactivator-1α in muscle and glucose transporter 4 in fat suggested that leucine improved peripheral fat oxidation and glucose transport. Leucine is able to improve peripheral glucose and lipid metabolism independent of appetite and weight regulation, suggesting its potential application in the management of metabolic disorders.

Obesity: considerations about etiology, metabolism, and the use of experimental models

Studies have been conducted in order to identify the main factors that contribute to the development of obesity. The role of genetics has also been extensively studied. However, the substantial augmentation of obesity prevalence in the last 20 years cannot be justified only by genetic alterations that, theoretically, would have occurred in such a short time. Thus, the difference in obesity prevalence in various population groups is also related to environmental factors, especially diet and the reduction of physical activity. These aspects, interacting or not with genetic factors, could explain the excess of body fat in large proportions worldwide. This article will focus on positive energy balance, high-fat diet, alteration in appetite control hormones, insulin resistance, amino acids metabolism, and the limitation of the experimental models to address this complex issue.

Exercise improves skeletal muscle insulin resistance without reduced basal mTOR/S6K1 signaling in rats fed a high-fat diet

Exercise improves high-fat diet (HFD)-induced skeletal muscle insulin resistance, but the mechanism is unresolved. This study aims to explore whether the improvement in response to exercise is associated with mTOR/S6K1 signaling and whether the signaling changes are muscle-specific. Male SD rats (150-180 g) were used for this study. After the experimental period, 6 weeks of exercise improved HFD-impaired intraperitoneal glucose tolerance and insulin-stimulated 2-deoxyglucose uptake in soleus (SOL) and extensor digitorum longus (EDL) muscles. Furthermore, 6 weeks of the HFD resulted in a reduced type I fiber ratio of SOL, an increased type I ratio of EDL, and a reduced fiber size of EDL, whereas exercise increased type I fiber ratio of SOL as well as type I fiber cross-sectional areas of EDL. However, the HFD had a main effect on basal cytosolic phosphorylation of S6K1 on Thr(389) content in SOL, which was also influenced by a significant interaction between the diet and exercise in EDL. Exercise had no direct effect on the basal phosphorylation of Akt on Ser(473), mTOR on Ser(2448), S6K1 on Thr(389) content in SOL. On the contrary, exercise prevented HFD-induced decrease in basal phosphorylation of S6K1 on Thr(389) content in EDL. These results indicate that 6 weeks of HFD and exercise lead to alterations in fiber type shift, fiber size, and basal phosphorylation of S6K1 on Thr(389) content in a muscle-specific pattern. Exercise prevents HFD-induced skeletal muscle insulin resistance, which is not associated with a reduced basal phosphorylation of mTOR/S6K1 alteration in the muscles.

AICAR treatment for 14 days normalizes obesity-induced dysregulation of TORC1 signaling and translational capacity in fasted skeletal muscle

The aim of this study was to determine the effect of 14 days of 5-aminoimidazole-4-carboxamide-1β-4-ribofuranoside (AICAR) treatment on mammalian target of rapamycin (mTOR) signaling and mTOR-regulated processes (i.e., translation initiation) in obese mouse skeletal muscle. Our hypothesis was that daily treatment (14 days) with AICAR would normalize obesity-induced alterations in skeletal muscle mTOR signaling and mTOR-regulated processes to lean levels and positively affect muscle mass. Fourteen-week-old male, lean (L; 31.3 g body wt) wild-type and ob/ob (O; 59.6 g body wt) mice were injected with the AMP-activated kinase (AMPK) activator AICAR (A) at 0.5 mg·g body wt(-1)·day(-1) or saline control (C) for 14 days. At 24 h after the last injection (including a 12-h fast), all mice were killed, and the plantar flexor complex muscle (gastrocnemius, soleus, and plantaris) was excised for analysis. Muscle mass was lower in OC (159 ± 12 mg) than LC, LA, and OA (176 ± 10, 178 ± 9, and 166 ± 16 mg, respectively) mice, independent of a body weight change. A decrease in obese muscle mass corresponded with higher muscle cross section staining intensity for lipid and glycogen, higher blood glucose and insulin levels, and lower nuclear-enriched fractions for peroxisome proliferator-activated receptor-γ coactivator-1α protein expression in OC skeletal muscle, which was normalized with AICAR treatment. AMPK and acetyl-cocarboxylase phosphorylation was reduced in OC mice and augmented by AICAR treatment in OA mice. Conversely, OC mice displayed higher activation of downstream targets (S6 kinase-1 and ribosomal protein S6) of mTOR and lower raptor-associated mTOR than LC mice, which were reciprocally altered after 14 days of AICAR treatment. Dysregulation of translational capacity was improved in OA mice, as assessed by sucrose density gradient fractionation of ribosomes, total and ribosome-associated RNA content, eukaryotic initiation factor 4F complex formation, and eukaryotic initiation factor 4G phosphorylation. These data show that short-term (14 days) AMPK agonist treatment augments regulatory processes in atrophic obese mouse skeletal muscle through the normalization of mTOR signaling and mRNA translation closer to lean levels.