Skeletal muscle inflammation is not responsible for the rapid impairment in adiponectin response with high-fat feeding in rats

Interscapular and Perivascular Brown Adipose Tissue Respond Differently to a Short-Term High-Fat Diet

Brown adipose tissue (BAT) function may depend on its anatomical location and developmental origin. Interscapular BAT (iBAT) regulates acute macronutrient metabolism, whilst perivascular BAT (PVAT) regulates vascular function. Although phenotypically similar, whether these depots respond differently to acute nutrient excess is unclear. Given their distinct anatomical locations and developmental origins and we hypothesised that iBAT and PVAT would respond differently to brief period of nutrient excess. Sprague-Dawley rats aged 12 weeks (n=12) were fed either a standard (10% fat, n=6) or high fat diet (HFD: 45% fat, n=6) for 72h and housed at thermoneutrality. Following an assessment of whole body physiology, fat was collected from both depots for analysis of gene expression and the proteome. HFD consumption for 72h induced rapid weight gain (c. 2.6%) and reduced serum non-esterified fatty acids (NEFA) with no change in either total adipose or depot mass. In iBAT, an upregulation of genes involved in insulin signalling and lipid metabolism was accompanied by enrichment of lipid-related processes and functions, plus glucagon and peroxisome proliferator-activated receptor (PPAR) signalling pathways. In PVAT, HFD induced a pronounced down-regulation of multiple metabolic pathways which was accompanied with increased abundance of proteins involved in apoptosis (e.g., Hdgf and Ywaq) and toll-like receptor signalling (Ube2n). There was also an enrichment of DNA-related processes and functions (e.g., nucleosome assembly and histone exchange) and RNA degradation and cell adhesion pathways. In conclusion, we show that iBAT and PVAT elicit divergent responses to short-term nutrient excess highlighting early adaptations in these depots before changes in fat mass.

Muscle-specific overexpression of AdipoR1 or AdipoR2 gives rise to common and discrete local effects whilst AdipoR2 promotes additional systemic effects

Hypoadiponectinemia and adiponectin resistance are implicated in the aetiology of obesity-related cardiometabolic disorders, hence represent a potential therapeutic axis. Here we characterised the effects of in vivo electrotransfer-mediated overexpression of the adiponectin receptors, AdipoR1 or AdipoR2, into tibialis anterior muscle (TAM) of lean or obese mice. In lean mice, TAM-specific overexpression of AdipoR1 (^TAMR1) or AdipoR2 (^TAMR2) increased phosphorylation of AMPK, AKT and ERK and expression of the insulin responsive glucose transporter glut4. In contrast, only ^TAMR2 increased pparα and a target gene acox1. These effects were decreased in obese mice despite no reduction in circulating adiponectin levels. ^TAMR2 also increased expression of adipoQ in TAM of lean and obese mice. Furthermore, in obese mice ^TAMR2 promoted systemic effects including; decreased weight gain; reduced epididymal fat mass and inflammation; increased epididymal adipoQ expression; increased circulating adiponectin. Collectively, these results demonstrate that AdipoR1 and AdipoR2 exhibit overlapping and distinct effects in skeletal muscle consistent with enhanced adiponectin sensitivity but these appear insufficient to ameliorate established obesity-induced adiponectin resistance. We also identify systemic effects upon ^TAMR2 in obese mice and postulate these are mediated by altered myokine production. Further studies are warranted to investigate this possibility which may reveal novel therapeutic approaches.

Skeletal muscle glucose metabolism and inflammation in the development of the metabolic syndrome

Insulin resistance and metabolic dysfunction in skeletal muscle play a major role in the development of the metabolic syndrome and type 2 diabetes. Numerous mechanisms have been proposed to explain the pathophysiology of obesity-linked metabolic dysfunction and this review will focus on the contributing role of adiponectin and inflammation. The beneficial effects of adiponectin on both insulin action and inflammation are now well documented and will be reviewed. More recent work provided new insights into adiponectin signaling mechanisms. The development of strategies to mimic adiponectin action holds promise that adiponectin-based compounds may translate into effective therapeutic applications. We will also discussed the novel role of long chain ω-3 PUFA-derived resolution mediators, which in addition to resolving inflammation, can also exert glucoregulatory effects in models of obesity and insulin resistance. We will focus on one resolution mediator, protectin DX (PDX), which was recently shown to act as a muscle interleukin-6 secretagogue. PDX and its isomer PD1 also enhance adiponectin expression and action. Ultimately, it is via a better understanding the molecular mechanisms of action via which inflammation, insulin resistance and metabolic dysfunction occur in skeletal muscle, and also how they crosstalk with each other, that we can generate new and improved therapies for obesity-linked metabolic complications.

Adiponectin action in skeletal muscle

The beneficial metabolic effects of adiponectin which confer insulin-sensitizing and anti-diabetic effects are well established. Skeletal muscle is an important target tissue for adiponectin where it regulates glucose and fatty acid metabolism directly and via insulin sensitizing effects. Cell surface receptors and the intracellular signaling events via which adiponectin orchestrates metabolism are now becoming well characterized. The initially accepted dogma of adiponectin action was that the physiological effects were mediated via endocrine effects of adipose-derived adiponectin. However, in recent years it has been established that skeletal muscle can also produce and secrete adiponectin that can elicit important functional effects. There is evidence that skeletal muscle adiponectin resistance may develop in obesity and play a role in the pathogenesis of diabetes. In summary, adiponectin acting in an autocrine and endocrine manner has important metabolic and insulin sensitizing effects on skeletal muscle which contribute to the overall anti-diabetic outcome of adiponectin action.

Acute exposure to high-fat diets increases hepatic expression of genes related to cell repair and remodeling in female rats

High-fat diets (HFD) promote the development of both obesity and fatty liver disease through the up-regulation of hepatic lipogenesis. Insulin resistance, a hallmark of both conditions, causes dysfunctional fuel partitioning and increases in lipogenesis. Recent work has demonstrated that systemic insulin resistance occurs in as little as the first 72 hours of an HFD, suggesting the potential for hepatic disruption with HFD at this time point. The current study sought to determine differences in expression of lipogenic genes between sexes in 3-month-old male and female Long-Evans rats after 72 hours of a 40% HFD or a 17% fat (chow) diet. Owing to the response of estrogen on hepatic signaling, we hypothesized that a sexual dimorphic response would occur in the expression of lipogenic enzymes, inflammatory cytokines, apoptotic, and cell repair and remodeling genes. Both sexes consumed more energy when fed an HFD compared with their low fat-fed controls. However, only the males fed the HFD had a significant increase in body fat. Regardless of sex, HFD caused down-regulation of lipogenic and inflammatory genes. Interestingly, females fed an HFD had up-regulated expression of apoptotic and cell repair-related genes compared with the males. This may suggest that females are more responsive to the acute hepatic injury effects caused by HFDs. In summary, neither male nor female rats displayed disrupted hepatic metabolic pathways after 72 hours of the HFD treatment. In addition, female rats appear to have protection from increases in fat deposition, possibly due to increased caloric expenditure; male rats fed an HFD were less active, as demonstrated by distance traveled in their home cage.

Fish oil prevents high-saturated fat diet-induced impairments in adiponectin and insulin response in rodent soleus muscle

High saturated fatty acid (SFA) diets contribute to the development of insulin resistance, whereas fish oil-derived n-3 polyunsaturated fatty acids (PUFA) increase the secretion of adiponectin (Ad), an adipocyte-derived protein that stimulates fatty acid oxidation (FAO) and improves skeletal muscle insulin response. We sought to determine whether fish oil could prevent and/or restore high SFA diet-induced impairments in Ad and insulin response in soleus muscle. Sprague-Dawley rats were fed 1) a low-fat control diet (CON group), 2) high-SFA diet (SFA group), or 3) high SFA with n-3 PUFA diet (SFA/n-3 PUFA group). At 4 wk, CON and SFA/n-3 PUFA animals were terminated, and SFA animals were either terminated or fed SFA or SFA/n-3 PUFA for an additional 2 or 4 wk. The effect of diet on Ad-stimulated FAO, insulin-stimulated glucose transport, and expression of Ad, insulin and inflammatory signaling proteins was determined in the soleus muscle. Ad stimulated FAO in CON and 4 wk SFA/n-3 PUFA (+36%, +39%, respectively P ≤ 0.05) only. Insulin increased glucose transport in CON, 4 wk SFA/n-3 PUFA, and 4 wk SFA + 4 wk SFA/n-3 PUFA (+82%, +33%, +25%, respectively P ≤ 0.05); this effect was lost in all other groups. TLR4 expression was increased with 4 wk of SFA feeding (+24%, P ≤ 0.05), and this was prevented in 4 wk SFA/n-3 PUFA. Suppressor of cytokine signaling-3 expression was increased in SFA and SFA/n-3 PUFA (+33 and +18%, respectively, P ≤ 0.05). Our results demonstrate that fish oil can prevent high SFA diet-induced impairments in both Ad and insulin response in soleus muscle.

Insulin-sensitizing properties of adiponectin

Adiponectin administration improves glucose tolerance in rodents. This is due to both reductions in hepatic glucose production, and likely improved insulin stimulated glucose disposal in skeletal muscle. Adiponectin's effects in both liver and muscle are believed to be due in large part to AMP-activated protein kinase (AMPK) activation, resulting in a reduction in hepatic gluconeogenic enzymes and increased fatty acid oxidation and reduced ectopic lipid deposition in muscle. In addition, adiponectin can robustly stimulate mitochondrial biogenesis, at least in muscle, and this appears to be due to AMPK-independent mechanisms. Various treatments successful at improving insulin response (thiazolidinediones (TZDs), n-3 polyunsaturated fatty acid (PUFA) supplementation) also stimulate adiponectin production. Obesity and insulin resistance are often characterized by both a state of resistance to adiponectin (both liver and muscle), as well as a reduction in total circulating adiponectin concentrations. The mechanisms underlying the impaired response of muscle and liver to adiponectin have not been clearly elucidated. Surprisingly, the significance of adiponectin resistance, at least in muscle, is not entirely clear. While the development of adiponectin resistance precedes intramuscular lipid accumulation and impaired insulin response in high-fat fed rodents, the restoration of adiponectin response does not appear to be necessary in order to restore insulin response in muscle. Further research examining the cellular mechanisms underlying the development of adiponectin resistance, and the importance of treating this, needs to be conducted.

Lifestyle factors increasing adiponectin synthesis and secretion

Adiponectin is an anti-inflammatory adipokine released from adipose tissue that is known to exert insulin-sensitizing effects in skeletal muscle and liver. Given that the secretion of adiponectin is impaired in obesity and related pathologies, strategies to enhance its synthesis and secretion are of interest. There is evidence that several lifestyle factors, including consumption of dietary long-chain n-3 PUFA, TZD administration, and weight loss can increase adiponectin synthesis and secretion. The effect of chronic exercise, independent of weight loss, is variable and less convincing. Potential mechanisms by which such lifestyle factors exert their favorable effects on adiponectin include activation of PPARγ and AMPK, regulation of posttranslational modifications, and changes in adipose tissue morphology and macrophage infiltration. As a clear role for adiponectin in mitigating obesity-related impairments in lipid metabolism and insulin sensitivity is evident, further research investigating factors that enhance adiponectin synthesis and secretion is distinctly warranted.

Nutritional and hormonal modulation of adiponectin and its receptors adipoR1 and adipoR2

Adiponectin is the most abundant plasma protein synthesized mostly by adipose tissue and is an insulin-sensitive hormone, playing a central role in glucose and lipid metabolism. Adiponectin effects are mediated via two receptors, adipoR1 and adipoR2. Several hormones and diet components that are involved in insulin resistance may impair insulin sensitivity at least in part by decreasing adiponectin and adiponectin receptors. Adiponectin expression and serum levels are associated with the amount and type of fatty acids and carbohydrate consumed. Other food items, such as vitamins, alcohol, sodium, green tea, and coffee, have been reported to modify adiponectin levels. Several hormones, including testosterone, estrogen, prolactin, glucocorticoids, catecholamines, and growth hormone, have been shown to inhibit adiponectin production, but the studies are still controversial. Even so, adiponectin is a potential therapeutic target in the treatment of diabetes mellitus and other diseases associated with hypoadiponectinemia.

Hydrogenated fat intake during pregnancy and lactation caused increase in TRAF-6 and reduced AdipoR1 in white adipose tissue, but not in muscle of 21 days old offspring rats

Background

Although lipids transfer through placenta is very limited, modification in dietary fatty acids can lead to implications in fetal and postnatal development. Trans fatty acid (TFA) intake during gestation and lactation have been reported to promote dyslipidemia and increase in pro- inflammatory adipokines in offspring. The aim of this study was to evaluate whether the alterations on pro-inflammatory cytokines and dyslipidemia observed previously in 21-d-old offspring of rats fed a diet containing hydrogenated vegetable fat during gestation and lactation were related to alterations in TLR-4, TRAF-6 and adipo-R1 receptor in white adipose tissue and muscle. On the first day of gestation, rats were randomly divided into two groups: (C) received a control diet, and (T) received a diet enriched with hydrogenated vegetable fat, rich in trans fatty acids. The diets were maintained throughout gestation and lactation. Each mother was given eight male pups. On the 21st day of life the offspring were killed. Blood, soleus and extensor digital longus (EDL) muscles, and retroperitoneal (RET) white adipose tissue were collected.

Results

21-d-old of T rats had higher serum triacylglycerols, cholesterol, and insulin. The Adipo R1 protein expression was lower in RET and higher in EDL of T group than C. TLR-4 protein content in all studied tissues were similar between groups, the same was verified in TRAF-6 protein expression in soleus and EDL. However, TRAF-6 protein expression in RET was higher in T than C.

Conclusion

These results demonstrated that maternal ingestion of hydrogenated vegetable fat rich in TFAs during gestation and lactation decrease in Adipo R1 protein expression and increase in TRAF-6 protein expression in retroperitoneal adipose tissue, but not in skeletal muscle, which could contributed for hyperinsulinemia and dyslipidemia observed in their 21-d-old offspring.

High-fat diet and glucocorticoid treatment cause hyperglycemia associated with adiponectin receptor alterations

BACKGROUND

Adiponectin is the most abundant plasma protein synthesized for the most part in adipose tissue, and it is an insulin-sensitive hormone, playing a central role in glucose and lipid metabolism. In addition, it increases fatty acid oxidation in the muscle and potentiates insulin inhibition of hepatic gluconeogenesis. Two adiponectin receptors have been identified: AdipoR1 is the major receptor expressed in skeletal muscle, whereas AdipoR2 is mainly expressed in liver. Consumption of high levels of dietary fat is thought to be a major factor in the promotion of obesity and insulin resistance. Excessive levels of cortisol are characterized by the symptoms of abdominal obesity, hypertension, glucose intolerance or diabetes and dyslipidemia; of note, all of these features are shared by the condition of insulin resistance. Although it has been shown that glucocorticoids inhibit adiponectin expression in vitro and in vivo, little is known about the regulation of adiponectin receptors. The link between glucocorticoids and insulin resistance may involve the adiponectin receptors and adrenalectomy might play a role not only in regulate expression and secretion of adiponectin, as well regulate the respective receptors in several tissues.

RESULTS

Feeding of a high-fat diet increased serum glucose levels and decreased adiponectin and adipoR2 mRNA expression in subcutaneous and retroperitoneal adipose tissues, respectively. Moreover, it increased both adipoR1 and adipoR2 mRNA levels in muscle and adipoR2 protein levels in liver. Adrenalectomy combined with the synthetic glucocorticoid dexamethasone treatment resulted in increased glucose and insulin levels, decreased serum adiponectin levels, reduced adiponectin mRNA in epididymal adipose tissue, reduction of adipoR2 mRNA by 7-fold in muscle and reduced adipoR1 and adipoR2 protein levels in muscle. Adrenalectomy alone increased adiponectin mRNA expression 3-fold in subcutaneous adipose tissue and reduced adipoR2 mRNA expression 2-fold in liver.

CONCLUSION

Hyperglycemia as a result of a high-fat diet is associated with an increase in the expression of the adiponectin receptors in muscle. An excess of glucocorticoids, rather than their absence, increase glucose and insulin and decrease adiponectin levels.