Diet-induced obesity impairs muscle satellite cell activation and muscle repair through alterations in hepatocyte growth factor signaling.

A healthy skeletal muscle mass is essential in attenuating the complications of obesity. Importantly, healthy muscle function is maintained through adequate repair following overuse and injury. The purpose of this study was to investigate the impact of diet-induced obesity (DIO) on skeletal muscle repair and the functionality of the muscle satellite cell (SC) population. Male C57BL/6J mice were fed a standard chow or high-fat diet (60% kcal fat; DIO) for 8 weeks. Muscles from DIO mice subjected to cardiotoxin injury displayed attenuated muscle regeneration, as indicated by prolonged necrosis, delayed expression of MyoD and Myogenin, elevated collagen content, and persistent embryonic myosin heavy chain expression. While no significant differences in SC content were observed, SCs from DIO muscles did not activate normally nor did they respond to exogenous hepatocyte growth factor (HGF) despite similar receptor (cMet) density. Furthermore, HGF release from crushed muscle was significantly less than that from muscles of chow fed mice. This study demonstrates that deficits in muscle repair are present in DIO, and the impairments in the functionality of the muscle SC population as a result of altered HGF/c-met signaling are contributors to the delayed regeneration.

Glucagon-Like Peptide 2 (GLP-2) Stimulates Postprandial Chylomicron Production and Postabsorptive Release of Intestinal Triglyceride Storage Pools via Induction of Nitric Oxide Signaling in Male Hamsters and Mice.

The intestinal overproduction of apolipoprotein B48 (apoB48)-containing chylomicron particles is a common feature of diabetic dyslipidemia and contributes to cardiovascular risk in insulin resistant states. We previously reported that glucagon-like peptide-2 (GLP-2) is a key endocrine stimulator of enterocyte fat absorption and chylomicron output in the postprandial state. GLP-2's stimulatory effect on chylomicron production in the postabsorptive state has been confirmed in human studies. The mechanism by which GLP-2 regulates chylomicron production is unclear, because its receptor is not expressed on enterocytes. We provide evidence for a key role of nitric oxide (NO) in mediating the stimulatory effects of GLP-2 during the postprandial and postabsorptive periods. Intestinal chylomicron production was assessed in GLP-2-treated hamsters administered the pan-specific NO synthase (NOS) inhibitor L-N(G)-nitroarginine methyl ester (L-NAME), and in GLP-2-treated endothelial NOS knockout mice. L-NAME blocked GLP-2-stimulated apoB48 secretion and reduced triglycerides (TGs) in the TG-rich lipoprotein (TRL) fraction of the plasma in the postprandial state. Endothelial NOS-deficient mice were resistant to GLP-2 stimulation and secreted fewer large apoB48-particles. When TG storage pools were allowed to accumulate, L-NAME mitigated the GLP-2-mediated increase in TRL-TG, suggesting that NO is required for early mobilization and secretion of stored TG and preformed chylomicrons. Importantly, the NO donor S-nitroso-L-glutathione was able to elicit an increase in TRL-TG in vivo and stimulate chylomicron release in vitro in primary enterocytes. We describe a novel role for GLP-2-mediated NO-signaling as a critical regulator of intestinal lipid handling and a potential contributor to postprandial dyslipidemia.

Early oxidative shifts in mouse skeletal muscle morphology with high-fat diet consumption do not lead to functional improvements.

Short-term consumption of a high-fat diet (HFD) can result in an oxidative shift in adult skeletal muscle. However, the impact of HFD on young, growing muscle is largely unknown. Thus, 4-week-old mice were randomly divided into sedentary HFD (60% kcal from fat), sedentary standard chow (control), or exercise-trained standard chow. Tibialis anterior (TA) and soleus muscles were examined for morphological and functional changes after 3 weeks. HFD consumption increased body and epididymal fat mass and induced whole body glucose intolerance versus control mice. Compared to controls, both HFD and exercise-trained TA muscles displayed a greater proportion of oxidative fibers and a trend for an increased succinate dehydrogenase (SDH) content. The soleus also displayed an oxidative shift with increased SDH content in HFD mice. Despite the aforementioned changes, palmitate oxidation rates were not different between groups. To determine if the adaptive changes with HFD manifest as a functional improvement, all groups performed pre- and postexperiment aerobic exercise tests. As expected, exercise-trained mice improved significantly compared to controls, however, no improvement was observed in HFD mice. Interestingly, capillary density was lower in HFD muscles; a finding which may contribute to the lack of functional differences seen with HFD despite the oxidative shift in skeletal muscle morphology. Taken together, our data demonstrate that young, growing muscle exhibits early oxidative shifts in response to a HFD, but these changes do not translate to functional benefits in palmitate oxidation, muscle fatigue resistance, or whole body exercise capacity.

Enhanced lipid oxidation and maintenance of muscle insulin sensitivity despite glucose intolerance in a diet-induced obesity mouse model.

BACKGROUND: Diet-induced obesity is a rising health concern which can lead to the development of glucose intolerance and muscle insulin resistance and, ultimately, type II diabetes mellitus. This research investigates the associations between glucose intolerance or muscle insulin resistance and tissue specific changes during the progression of diet-induced obesity. METHODOLOGY: C57BL/6J mice were fed a normal or high-fat diet (HFD; 60% kcal fat) for 3 or 8 weeks. Disease progression was monitored by measurements of body/tissue mass changes, glucose and insulin tolerance tests, and ex vivo glucose uptake in intact muscles. Lipid metabolism was analyzed using metabolic chambers and ex vivo palmitate assays in intact muscles. Skeletal muscle, liver and adipose tissues were analyzed for changes in inflammatory gene expression. Plasma was analyzed for insulin levels and inflammatory proteins. Histological techniques were used on muscle and liver cryosections to assess metabolic and morphological changes. PRINCIPAL FINDINGS/CONCLUSIONS: A rapid shift in whole body metabolism towards lipids was observed with HFD. Following 3 weeks of HFD, elevated total lipid oxidation and an oxidative fiber type shift had occurred in the skeletal muscle, which we propose was responsible for delaying intramyocellular lipid accumulation and maintaining muscle's insulin sensitivity. Glucose intolerance was present after three weeks of HFD and was associated with an enlarged adipose tissue depot, adipose tissue inflammation and excess hepatic lipids, but not hepatic inflammation. Furthermore, HFD did not significantly increase systemic or muscle inflammation after 3 or 8 weeks of HFD suggesting that early diet-induced obesity does not cause inflammation throughout the whole body. Overall these findings indicate skeletal muscle did not contribute to the development of HFD-induced impairments in whole-body glucose tolerance following 3 weeks of HFD.

Intestinal SR-BI is upregulated in insulin-resistant states and is associated with overproduction of intestinal apoB48-containing lipoproteins.

Intestinal lipid dysregulation is a common feature of insulin-resistant states. The present study investigated alterations in gene expression of key proteins involved in the active absorption of dietary fat and cholesterol in response to development of insulin resistance. Studies were conducted in two diet-induced animal models of insulin resistance: fructose-fed hamster and high-fat-fed mouse. Changes in the mRNA abundance of lipid transporters, adenosine triphosphate cassette (ABC) G5, ABCG8, FA-CoA ligase fatty acid translocase P4, Niemann-Pick C1-Like1 (NPC1L1), fatty acid transport protein 4 (FATP4), and Scavenger Receptor Class B Type I (SR-BI), were assessed in intestinal fragments (duodenum, jejunum, and ileum) using quantitative real-time PCR. Of all the transporters evaluated, SR-B1 showed the most significant changes in both animal models examined. A marked stimulation of SR-B1 expression was observed in all intestinal segments examined in both insulin-resistant animal models. The link between SR-BI expression and intestinal lipoprotein production was then examined in the Caco-2 cell model. SR-B1 overexpression in Caco-2 cells increased apolipoprotein B (apoB) 100 and apoB48 secretion, whereas RNAi knock down of SR-B1 decreased secretion of both apoB100 and apoB48. We also observed changes in subcellular distribution of SR-B1 in response to exogenous lipid and insulin. Confocal microscopy revealed marked changes in SR-BI subcellular distribution in response to both exogenous lipids (oleate) and insulin. In summary, marked stimulation of intestinal SR-BI occurs in vivo in animal models of diet-induced insulin resistance, and modulation of SR-BI in vitro regulates production of apoB-containing lipoprotein particles. We postulate that apical and/or basolateral SR-BI may play an important role in intestinal chylomicron production and may contribute to chylomicron overproduction normally observed in insulin-resistant states.

Volume regulation in mammalian skeletal muscle: the role of sodium-potassium-chloride cotransporters during exposure to hypertonic solutions.

Controversy exists as to whether mammalian skeletal muscle is capable of volume regulation in response to changes in extracellular osmolarity despite evidence that muscle fibres have the required ion transport mechanisms to transport solute and water in situ. We addressed this issue by studying the ability of skeletal muscle to regulate volume during periods of induced hyperosmotic stress using single, mouse extensor digitorum longus (EDL) muscle fibres and intact muscle (soleus and EDL). Fibres and intact muscles were loaded with the fluorophore, calcein, and the change in muscle fluorescence and width (single fibres only) used as a metric of volume change. We hypothesized that skeletal muscle exposed to increased extracellular osmolarity would elicit initial cellular shrinkage followed by a regulatory volume increase (RVI) with the RVI dependent on the sodium­potassium­chloride cotransporter (NKCC). We found that single fibres exposed to a 35% increase in extracellular osmolarity demonstrated a rapid, initial 27­32% decrease in cell volume followed by a RVI which took 10-20 min and returned cell volume to 90­110% of pre-stimulus values. Within intact muscle, exposure to increased extracellular osmolarity of varying degrees also induced a rapid, initial shrinkage followed by a gradual RVI, with a greater rate of initial cell shrinkage and a longer time for RVI to occur with increasing extracellular tonicities. Furthermore, RVI was significantly faster in slow-twitch soleus than fast-twitch EDL. Pre-treatment of muscle with bumetanide (NKCC inhibitor) or ouabain (Na+,K+-ATPase inhibitor), increased the initial volume loss and impaired the RVI response to increased extracellular osmolarity indicating that the NKCC is a primary contributor to volume regulation in skeletal muscle. It is concluded that mouse skeletal muscle initially loses volume then exhibits a RVI when exposed to increases in extracellular osmolarity. The rate of RVI is dependent on the degree of change in extracellular osmolarity, is muscle specific, and is dependent on the functioning of the NKCC and Na+, K+-ATPase.