Aims/hypothesis
Aerobic exercise increases muscle glucose and improves insulin action through numerous pathways, including activation of Ca
2+
/calmodulin-dependent protein kinases (CAMKs) and ...peroxisome proliferator γ coactivator 1α (PGC-1α). While overexpression of PGC-1α increases muscle mitochondrial content and oxidative type I fibres, it does not improve insulin action. Activation of CAMK4 also increases the content of type I muscle fibres, PGC-1α level and mitochondrial content. However, it remains unknown whether CAMK4 activation improves insulin action on glucose metabolism in vivo.
Methods
The effects of CAMK4 activation on skeletal muscle insulin action were quantified using transgenic mice with a truncated and constitutively active form of CAMK4 (CAMK4
●
) in skeletal muscle. Tissue-specific insulin sensitivity was assessed in vivo using a hyperinsulinaemic–euglycaemic clamp and isotopic measurements of glucose metabolism.
Results
The rate of insulin-stimulated whole-body glucose uptake was increased by ∼25% in CAMK4
●
mice. This was largely attributed to an increase of ∼60% in insulin-stimulated glucose uptake in the quadriceps, the largest hindlimb muscle. These changes were associated with improvements in insulin signalling, as reflected by increased phosphorylation of Akt and its substrates and an increase in the level of GLUT4 protein. In addition, there were extramuscular effects: CAMK4
●
mice had improved hepatic and adipose insulin action. These pleiotropic effects were associated with increased levels of PGC-1α-related myokines in CAMK4
●
skeletal muscle.
Conclusions/interpretation
Activation of CAMK4 enhances mitochondrial biogenesis in skeletal muscle while also coordinating improvements in whole-body insulin-mediated glucose.
Fatty acid amide hydrolase (FAAH) knockout mice are prone to excess energy storage and adiposity, whereas mutations in FAAH are associated with obesity in humans. However, the molecular mechanism by ...which FAAH affects energy expenditure (EE) remains unknown. Here we show that reduced energy expenditure in FAAH-/- mice could be attributed to decreased circulating triiodothyronine and thyroxine concentrations secondary to reduced mRNA expression of both pituitary thyroid-stimulating hormone and hypothalamic thyrotropin-releasing hormone. These reductions in the hypothalamic-pituitary-thyroid axis were associated with activation of hypothalamic peroxisome proliferating-activated receptor γ (PPARγ), and increased hypothalamic deiodinase 2 expression. Infusion of NAEs (anandamide and palmitoylethanolamide) recapitulated increases in PPARγ-mediated decreases in EE. FAAH-/- mice were also prone to diet-induced hepatic insulin resistance, which could be attributed to increased hepatic diacylglycerol content and protein kinase Ce activation. Our data indicate that FAAH deletion, and the resulting increases in NAEs, predispose mice to ectopic lipid storage and hepatic insulin resistance by promoting centrally mediated hypothyroidism.
Type 2 diabetes is characterized by insulin resistance and a gradual loss of pancreatic beta cell mass and function
. Currently, there are no therapies proven to prevent beta cell loss and some, ...namely insulin secretagogues, have been linked to accelerated beta cell failure, thereby limiting their use in type 2 diabetes
. The adipokine adipsin/complement factor D controls the alternative complement pathway and generation of complement component C3a, which acts to augment beta cell insulin secretion
. In contrast to other insulin secretagogues, we show that chronic replenishment of adipsin in diabetic db/db mice ameliorates hyperglycemia and increases insulin levels while preserving beta cells by blocking dedifferentiation and death. Mechanistically, we find that adipsin/C3a decreases the phosphatase Dusp26; forced expression of Dusp26 in beta cells decreases expression of core beta cell identity genes and sensitizes to cell death. In contrast, pharmacological inhibition of DUSP26 improves hyperglycemia in diabetic mice and protects human islet cells from cell death. Pertaining to human health, we show that higher concentrations of circulating adipsin are associated with a significantly lower risk of developing future diabetes among middle-aged adults after adjusting for body mass index (BMI). Collectively, these data suggest that adipsin/C3a and DUSP26-directed therapies may represent a novel approach to achieve beta cell health to treat and prevent type 2 diabetes.
Mitochondrial dysfunction is associated with many human diseases and results from mismatch of damage and repair over the life of the organelle. PARK2 is a ubiquitin E3 ligase that regulates ...mitophagy, a repair mechanism that selectively degrades damaged mitochondria. Deletion of PARK2 in multiple in vivo models results in susceptibility to stress-induced mitochondrial and cellular dysfunction. Surprisingly, Park2 knockout (KO) mice are protected from nutritional stress and do not develop obesity, hepatic steatosis or insulin resistance when fed a high-fat diet (HFD). However, these phenomena are casually related and the physiological basis for this phenotype is unknown. We therefore undertook a series of acute HFD studies to more completely understand the physiology of Park2 KO during nutritional stress. We find that intestinal lipid absorption is impaired in Park2 KO mice as evidenced by increased fecal lipids and reduced plasma triglycerides after intragastric fat challenge. Park2 KO mice developed hepatic steatosis in response to intravenous lipid infusion as well as during incubation of primary hepatocytes with fatty acids, suggesting that hepatic protection from nutritional stress was secondary to changes in energy balance due to altered intestinal triglyceride absorption. Park2 KO mice showed reduced adiposity after 1-wk HFD, as well as improved hepatic and peripheral insulin sensitivity. These studies suggest that changes in intestinal lipid absorption may play a primary role in protection from nutritional stress in Park2 KO mice by preventing HFD-induced weight gain and highlight the need for tissue-specific models to address the role of PARK2 during metabolic stress.
Exposure to the toxins methylene cyclopropyl acetic acid (MCPA) and methylene cyclopropyl glycine (MCPG) of unripe ackee and litchi fruit can lead to hypoglycemia and death; however, the molecular ...mechanisms by which MCPA and MCPG cause hypoglycemia have not been established
To determine the
mechanisms of action of these toxins, we infused them into conscious rodents and assessed rates of hepatic gluconeogenesis and ketogenesis, hepatic acyl-CoA and hepatic acetyl-CoA content, and hepatocellular energy charge. MCPG suppressed rates of hepatic β-oxidation as reflected by reductions in hepatic ketogenesis, reducing both short- and medium-chain hepatic acyl-CoA concentrations. Hepatic acetyl-CoA content decreased, and hepatic glucose production was inhibited. MCPA also suppressed β-oxidation of short-chain acyl-CoAs, rapidly inhibiting hepatic ketogenesis and hepatic glucose production, depleting hepatic acetyl-CoA content and ATP content, while increasing other short-chain acyl-CoAs. Utilizing a recently developed positional isotopomer NMR tracer analysis method, we demonstrated that MCPA-induced reductions in hepatic acetyl-CoA content were associated with a marked reduction of hepatic pyruvate carboxylase (PC) flux. Taken together, these data reveal the
mechanisms of action of MCPA and MCPG: the hypoglycemia associated with ingestion of these toxins can be ascribed mostly to MCPA- or MCPG-induced reductions in hepatic PC flux due to inhibition of β-oxidation of short-chain acyl-CoAs by MCPA or inhibition of both short- and medium-chain acyl-CoAs by MCPG with resultant reductions in hepatic acetyl-CoA content, with an additional contribution to hypoglycemia through reduced hepatic ATP stores by MCPA.
Several models of mice-fed high-fat diets have been used to trigger non-alcoholic steatohepatitis and some chemical substances, such as carbon tetrachloride. The present study aimed to evaluate the ...joint action of a high-fat diet and CCl4 in developing a short-term non-alcoholic steatohepatitis model. C57BL6/J mice were divided into two groups: standard diet-fed (SD), the high-fat diet-fed (HFD) and HFD + fructose-fed and carbon tetrachloride (HFD+CCl4). The animals fed with HFD+CCl4 presented increased lipid deposition compared with both SD and HFD mice. Plasma cholesterol was increased in animals from the HFD+CCl4 group compared with the SD and HFD groups, without significant differences between the SD and HFD groups. Plasma triglycerides showed no significant difference between the groups. The HFD+CCl4 animals had increased collagen deposition in the liver compared with both SD and HFD groups. Hydroxyproline was also increased in the HFD+CCl4 group. Liver enzymes, alanine aminotransferase and aspartate aminotransferase, were increased in the HFD+CCl4 group, compared with SD and HFD groups. Also, CCl4 was able to trigger an inflammatory process in the liver of HFD-fed animals by promoting an increase of ∼2 times in macrophage activity, ∼6 times in F4/80 gene expression, and pro-inflammatory cytokines (IL-1b and TNFa), in addition to an increase in inflammatory pathway protein phosphorylation (IKKbp). HFD e HFD+CCl4 animals increased glucose intolerance compared with SD mice, associated with reduced insulin-stimulated AKT activity in the liver. Therefore, our study has shown that short-term HFD feeding associated with fructose and CCl4 can trigger non-alcoholic steatohepatitis and cause damage to glucose metabolism.
Uncoupling protein 1 (UCP1) plays a central role in nonshivering thermogenesis in brown fat; however, its role in beige fat remains unclear. Here we report a robust UCP1-independent thermogenic ...mechanism in beige fat that involves enhanced ATP-dependent Ca
cycling by sarco/endoplasmic reticulum Ca
-ATPase 2b (SERCA2b) and ryanodine receptor 2 (RyR2). Inhibition of SERCA2b impairs UCP1-independent beige fat thermogenesis in humans and mice as well as in pigs, a species that lacks a functional UCP1 protein. Conversely, enhanced Ca
cycling by activation of α1- and/or β3-adrenergic receptors or the SERCA2b-RyR2 pathway stimulates UCP1-independent thermogenesis in beige adipocytes. In the absence of UCP1, beige fat dynamically expends glucose through enhanced glycolysis, tricarboxylic acid metabolism and pyruvate dehydrogenase activity for ATP-dependent thermogenesis through the SERCA2b pathway; beige fat thereby functions as a 'glucose sink' and improves glucose tolerance independently of body weight loss. Our study uncovers a noncanonical thermogenic mechanism through which beige fat controls whole-body energy homeostasis via Ca
cycling.
Key points
Oestrogen has been shown to play an important role in the regulation of metabolic homeostasis and insulin sensitivity in both human and rodent studies.
Insulin sensitivity is greater in ...premenopausal women compared with age‐matched men, and metabolism‐related cardiovascular diseases and type 2 diabetes are less frequent in these same women.
Both female and male mice treated with oestradiol are protected against obesity‐induced insulin resistance.
The protection against obesity‐induced insulin resistance is associated with reduced ectopic lipid content in liver and skeletal muscle.
These results were associated with increased insulin‐stimulated suppression of white adipose tissue lipolysis and reduced inflammation.
Oestrogen has been shown to play an important role in the regulation of metabolic homeostasis and insulin sensitivity in both human and rodent studies. Overall, females are protected against obesity‐induced insulin resistance; yet, the mechanisms responsible for this protection are not well understood. Therefore, the aim of the present work was to evaluate the underlying mechanism(s) by which female mice are protected against obesity‐induced insulin resistance compared with male mice. We studied male and female mice in age‐matched or body weight‐matched conditions. They were fed a high‐fat diet (HFD) or regular chow for 4 weeks. We also studied HFD male mice treated with oestradiol or vehicle. Both HFD female and HFD male mice treated with oestradiol displayed increased whole‐body insulin sensitivity, associated with reduction in ectopic hepatic and muscle lipid content compared to HFD male mice. Reductions in ectopic lipid content in these mice were associated with increased insulin‐stimulated suppression of white adipose tissue (WAT) lipolysis. Both HFD female and HFD male mice treated with oestradiol also displayed striking reductions in WAT inflammation, represented by reductions in plasma and adipose tissue tumour necrosis factor α and interleukin 6 concentrations. Taken together these data support the hypothesis that HFD female mice are protected from obesity‐induced insulin resistance due to oestradiol‐mediated reductions in WAT inflammation, leading to improved insulin‐mediated suppression of WAT lipolysis and reduced ectopic lipid content in liver and skeletal muscle.
Key points
Oestrogen has been shown to play an important role in the regulation of metabolic homeostasis and insulin sensitivity in both human and rodent studies.
Insulin sensitivity is greater in premenopausal women compared with age‐matched men, and metabolism‐related cardiovascular diseases and type 2 diabetes are less frequent in these same women.
Both female and male mice treated with oestradiol are protected against obesity‐induced insulin resistance.
The protection against obesity‐induced insulin resistance is associated with reduced ectopic lipid content in liver and skeletal muscle.
These results were associated with increased insulin‐stimulated suppression of white adipose tissue lipolysis and reduced inflammation.
One of the consequences of the Western lifestyle and high-fat diet is non-alcoholic fatty liver disease (NAFLD) and its aggressive form, non-alcoholic steatohepatitis (NASH), which can progress to ...cirrhosis and hepatocellular carcinoma (HCC) and is rapidly becoming the leading cause of end-stage liver disease or liver transplantation. Currently, rodent NASH models lack significant aspects of the full NASH spectrum, representing a major problem for NASH research. Therefore, this work aimed to characterize a fast rodent model with all characteristic features of NASH. Eight-week-old male ApoE KO mice were fed with Western diet (WD), high fatty diet (HFD) or normal chow (Chow) for 7 weeks. Whole-body fat was increased by ~2 times in WD mice and HFD mice and was associated with increased glucose intolerance, hepatic triglycerides, and plasma ALT and plasma AST compared with Chow mice. WD mice also showed increased galectin-3 expression compared with Chow or HFD mice and increased plasma cholesterol compared with Chow mice. WD and HFD displayed increased hepatic fibrosis and increased F4/80 expression. WD mice also displayed increased levels of plasma MCP-1. Hepatic inflammatory markers were evaluated, and WD mice showed increased levels of TNF-α, MCP-1, IL-6 and IFN-γ. Taken together, these data demonstrated that the ApoE KO mouse fed with WD is a great model for NASH research, once it presents the fundamental parameters of the disease, including hepatic steatosis, fibrosis, inflammation, and metabolic syndrome.
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