Atrophy of White Adipose Tissue Accompanied with Decreased Insulin-Stimulated Glucose Uptake in Mice Lacking the Small GTPase Rac1 Specifically in Adipocytes

Insulin stimulates glucose uptake in adipose tissue and skeletal muscle by inducing plasma membrane translocation of the glucose transporter GLUT4. Although the small GTPase Rac1 is a key regulator downstream of phosphoinositide 3-kinase (PI3K) and the protein kinase Akt2 in skeletal muscle, it remains unclear whether Rac1 also regulates glucose uptake in white adipocytes. Herein, we investigated the physiological role of Rac1 in white adipocytes by employing adipocyte-specific rac1 knockout (adipo-rac1-KO) mice. Subcutaneous and epididymal white adipose tissues (WATs) in adipo-rac1-KO mice showed significant reductions in size and weight. Actually, white adipocytes lacking Rac1 were smaller than controls. Insulin-stimulated glucose uptake and GLUT4 translocation were abrogated in rac1-KO white adipocytes. On the other hand, GLUT4 translocation was augmented by constitutively activated PI3K or Akt2 in control, but not in rac1-KO, white adipocytes. Similarly, to skeletal muscle, the involvement of another small GTPase RalA downstream of Rac1 was demonstrated. In addition, mRNA levels of various lipogenic enzymes were down-regulated in rac1-KO white adipocytes. Collectively, these results suggest that Rac1 is implicated in insulin-dependent glucose uptake and lipogenesis in white adipocytes, and reduced insulin responsiveness due to the deficiency of Rac1 may be a likely explanation for atrophy of WATs.     

体外循环对兔骨骼肌葡萄糖转运蛋白4和胰岛葡萄糖转运蛋白2的影响

目的观察体外循环(Cardiopulmonary bypass,CPB)对兔骨骼肌葡萄糖转运蛋白4(Glucose transporter 4,GLUT4)和胰岛葡萄糖转运蛋白2(GLUT2)的影响,探讨CPB中高血糖的可能机制。方法取健康成年新西兰大白兔24只,随机分为正常对照组(N组)和CPB组(C组),每组12只。N组不行CPB,C组行CPB。两组分别于麻醉后即刻(T1)、主动脉阻断即刻(T2)、主动脉开放5min(T3)、主动脉开放35min(T4)和主动脉开放75min(T5)采集动脉血液,氧化酶法检测血糖水平,放免法检测胰岛素水平;免疫组织化学法检测主动脉开放5min和75min时骨骼肌总GLUT4和细胞膜上GLUT4的转位表达及开放75min时胰岛细胞GLUT2的表达。结果与T1比较,两组T2～T5的血糖和胰岛素水平均显著升高(P<0.05);C组T2～T5的血糖和胰岛素水平较N组显著升高(P<0.05);主动脉开放5min和75min时,C组骨骼肌总GLUT4和细胞膜上GLUT4的转位表达均显著下调(P<0.05);主动脉开放75min时,胰岛细胞GLUT2的表达显著上调(P<0.05);C组主动脉开放75min比5min时,骨骼肌总GLUT4和细胞膜上GLUT4的转位表达均显著上调(P<0.05)。结论兔CPB中高血糖可能与骨骼肌GLUT4表达下调和转位障碍有关。胰岛细胞GLUT2表达上调可能使胰岛素分泌增加,但增加的胰岛素并不能完全代偿高血糖。     

Insulin Resistance Is Not Sustained Following Denervation in Glycolytic Skeletal Muscle

Denervation rapidly induces insulin resistance (i.e., impairments in insulin-stimulated glucose uptake and signaling proteins) in skeletal muscle. Surprisingly, whether this metabolic derangement is long-lasting is presently not clear. The main goal of this study was to determine if insulin resistance is sustained in both oxidative soleus and glycolytic extensor digitorum longus (EDL) muscles following long-term (28 days) denervation. Mouse hindlimb muscles were denervated via unilateral sciatic nerve resection. Both soleus and EDL muscles atrophied ~40%. Strikingly, while denervation impaired submaximal insulin-stimulated [³H]-2-deoxyglucose uptake ~30% in the soleus, it enhanced submaximal (~120%) and maximal (~160%) insulin-stimulated glucose uptake in the EDL. To assess possible mechanism(s), immunoblots were performed. Denervation did not consistently alter insulin signaling (e.g., p-Akt (Thr308):Akt; p-TBC1D1 [phospho-Akt substrate (PAS)]:TBC1D1; or p-TBC1D4 (PAS):TBC1D4) in either muscle. However, denervation decreased glucose transporter 4 (GLUT4) levels ~65% in the soleus but increased them ~90% in the EDL. To assess the contribution of GLUT4 to the enhanced EDL muscle glucose uptake, muscle-specific GLUT4 knockout mice were examined. Loss of GLUT4 prevented the denervation-induced increase in insulin-stimulated glucose uptake. In conclusion, the denervation results sustained insulin resistance in the soleus but enhanced insulin sensitivity in the EDL due to increased GLUT4 protein levels.     

Decreased Glucocorticoid Signaling Potentiates Lipid-Induced Inflammation and Contributes to Insulin Resistance in the Skeletal Muscle of Fructose-Fed Male Rats Exposed to Stress

The modern lifestyle brings both excessive fructose consumption and daily exposure to stress which could lead to metabolic disturbances and type 2 diabetes. Muscles are important points of glucose and lipid metabolism, with a crucial role in the maintenance of systemic energy homeostasis. We investigated whether 9-week fructose-enriched diet, with and without exposure to 4-week unpredictable stress, disturbs insulin signaling in the skeletal muscle of male rats and evaluated potential contributory roles of muscle lipid metabolism, glucocorticoid signaling and inflammation. The combination of fructose-enriched diet and stress increased peroxisome proliferator-activated receptors-α and -δ and stimulated lipid uptake, lipolysis and β-oxidation in the muscle of fructose-fed stressed rats. Combination of treatment also decreased systemic insulin sensitivity judged by lower R-QUICKI, and lowered muscle protein content and stimulatory phosphorylations of insulin receptor supstrate-1 and Akt, as well as the level of 11β-hydroxysteroid dehydrogenase type 1 and glucocorticoid receptor. At the same time, increased levels of protein tyrosine phosphatase-1B, nuclear factor-κB, tumor necrosis factor-α, were observed in the muscle of fructose-fed stressed rats. Based on these results, we propose that decreased glucocorticoid signaling in the skeletal muscle can make a setting for lipid-induced inflammation and the development of insulin resistance in fructose-fed stressed rats.     

Molecular Mechanisms for the Regulation of Insulin-Stimulated Glucose Uptake by Small Guanosine Triphosphatases in Skeletal Muscle and Adipocytes

Insulin is a hormone that regulates the blood glucose level by stimulating various physiological responses in its target tissues. In skeletal muscle and adipose tissue, insulin promotes membrane trafficking of the glucose transporter GLUT4 from GLUT4 storage vesicles to the plasma membrane, thereby facilitating the uptake of glucose from the circulation. Detailed mechanisms underlying insulin-dependent intracellular signal transduction for glucose uptake remain largely unknown. In this article, I give an overview on the recently identified signaling network involving Rab, Ras, and Rho family small guanosine triphosphatases (GTPases) that regulates glucose uptake in insulin-responsive tissues. In particular, the regulatory mechanisms for these small GTPases and the cross-talk between protein kinase and small GTPase cascades are highlighted.     

The hypotriglyceridemic effect of dietary n−3 FA is associated with increased β-oxidation and reduced leptin expression

To study the mechanisms responsible for the hypotriglyceridemic effect of marine oils, we monitored the effects of high dietary intake of n-3 PUFA on hepatic and muscular beta-oxidation, plasma leptin concentration, leptin receptor gene expression, and in vivo insulin action. Two groups of male Wistar rats were fed either a high-fat diet [28% (w/w) of saturated fat] or a high-fat diet containing 10% n-3 PUFA and 18% saturated fat for 3 wk. The hypotriglyceridemic effect of n-3 PUFA was accompanied by increased hepatic oxidation of palmitoyl-CoA (125%, P < 0.005) and palmitoyl-L-carnitine (480%, P < 0.005). These findings were corroborated by raised carnitine palmitoyltransferase-2 activity (154%, P < 0.001) and mRNA levels (91%, P < 0.01) as well as by simultaneous elevation of hepatic peroxisomal acyl-CoA oxidase activity (144%, P < 0.01) and mRNA content (82%, P < 0.05). In contrast, hepatic carnitine palmitoyltransferase-1 activity remained unchanged despite a twofold increased mRNA level after n-3 PUFA feeding. Skeletal muscle FA oxidation was less affected by dietary n-3 PUFA, and the stimulatory effect was found only in peroxisomes. Dietary intake of n-3 PUFA was followed by increased acyl-CoA oxidase activity (48%, P < 0.05) and mRNA level (83%, P < 0.05) in skeletal muscle. The increased FA oxidation after n-3 PUFA supplementation of the high-fat diet was accompanied by lower plasma leptin concentration (-38%, P < 0.05) and leptin mRNA expression (-66%, P < 0.05) in retroperitoneal adipose tissue, and elevated hepatic mRNA level for the leptin receptor Ob-Ra (140%, P < 0.05). Supplementation of the high-fat diet with n-3 PUFA enhanced in vivo insulin sensitivity, as shown by normalization of the glucose infusion rate during euglycemic hyperinsulinemic clamp. Our results indicate that the hypotriglyceridemic effect of dietary n-3 PUFA is associated with stimulation of FA oxidation in the liver and to a smaller extent in skeletal muscle. This may ameliorate dyslipidemia, tissue lipid accumulation, and insulin action, in spite of decreased plasma leptin level and leptin mRNA in adipose tissue.     

Effect of sucrose and saturated-fat diets on mRNA levels of genes limiting muscle fatty acid and glucose supply in rats

In this study, we examined whether the increased availability of lipids in blood resulting from two types of diet manipulation regulated metabolic gene expression in the skeletal muscle of rats. Feeding for 4 wk on an isocaloric-sucrose or a hypercaloric-fat diet increased plasma TAG in the fed condition by increments of 70 and 40%, respectively, and increased fasting insulinemia (approximately 3-fold) compared with a starch diet. The fat diet impaired glucose tolerance and caused obesity, whereas sucrose-fed rats maintained their normal weight. We analyzed the expression of genes that regulate the exogenous FA supply (LPL, FAT/CD36, FATP1), synthesis (ACC1), glucose (GLUT4, GLUT1, HK2, GRAT1, glycogen phosphorylase) or glycerol (glycerol kinase) provision, or substrate choice for oxidation (PDK4) in gastrocnemius and soleus muscles at the end of the glucose tolerance test. LPL, FAT/CD36, FATP1, PDK4, and GLUT4 mRNA as well as glycogen phosphorylase and glycerol kinase activity levels in both muscles were unchanged by the diets. Increased mRNA levels of GLUT1 (1.6- and 2.6-fold, respectively) and GFAT1 (about 1.7-fold) in gastrocnemius, and of ACC1 (about 1.5-fold) in soleus, were found in both the sucrose and fat groups. In the fat group, HK2 mRNA was also higher (1.8-fold) in the gastrocnemius. Both sucrose and saturated-fat diets prompted hyperinsulinemia and hyperlipemia in rats. These metabolic disturbances did not alter the expression of LPL, FAT/CD36, FATP1, PDK4, and GLUT4 genes or glycogen phosphorylase and glycerol kinase activity levels in either analyzed muscle. Instead, they were linked to the coordinated upregulation in gastrocnemius of genes that govern glucose uptake and the hexosamine pathway, namely, GLUT1 and GFAT1, which might contribute to insulin resistance.     

Feeding Rhythm-Induced Hypothalamic Agouti-Related Protein Elevation via Glucocorticoids Leads to Insulin Resistance in Skeletal Muscle

Circadian phase shifts in peripheral clocks induced by changes in feeding rhythm often result in insulin resistance. However, whether the hypothalamic control system for energy metabolism is involved in the feeding rhythm-related development of insulin resistance is unknown. Here, we show the physiological significance and mechanism of the involvement of the agouti-related protein (AgRP) in evening feeding-associated alterations in insulin sensitivity. Evening feeding during the active dark period increased hypothalamic AgRP expression and skeletal muscle insulin resistance in mice. Inhibiting AgRP expression by administering an antisense oligo or a glucocorticoid receptor antagonist mitigated these effects. AgRP-producing neuron-specific glucocorticoid receptor-knockout (AgRP-GR-KO) mice had normal skeletal muscle insulin sensitivity even under evening feeding schedules. Hepatic vagotomy enhanced AgRP expression in the hypothalamus even during ad-lib feeding in wild-type mice but not in AgRP-GR-KO mice. The findings of this study indicate that feeding in the late active period may affect hypothalamic AgRP expression via glucocorticoids and induce skeletal muscle insulin resistance.     

Insulin-Dependent H2O2 Production Is Higher in Muscle Fibers of Mice Fed with a High-Fat Diet

Insulin resistance is defined as a reduced ability of insulin to stimulate glucose utilization. C57BL/6 mice fed with a high-fat diet (HFD) are a model of insulin resistance. In skeletal muscle, hydrogen peroxide (H₂O₂) produced by NADPH oxidase 2 (NOX2) is involved in signaling pathways triggered by insulin. We evaluated oxidative status in skeletal muscle fibers from insulin-resistant and control mice by determining H₂O₂ generation (HyPer probe), reduced-to-oxidized glutathione ratio and NOX2 expression. After eight weeks of HFD, insulin-dependent glucose uptake was impaired in skeletal muscle fibers when compared with control muscle fibers. Insulin-resistant mice showed increased insulin-stimulated H₂O₂ release and decreased reduced-to-oxidized glutathione ratio (GSH/GSSG). In addition, p47^phox and gp91^phox (NOX2 subunits) mRNA levels were also high (~3-fold in HFD mice compared to controls), while protein levels were 6.8- and 1.6-fold higher, respectively. Using apocynin (NOX2 inhibitor) during the HFD feeding period, the oxidative intracellular environment was diminished and skeletal muscle insulin-dependent glucose uptake restored. Our results indicate that insulin-resistant mice have increased H₂O₂ release upon insulin stimulation when compared with control animals, which appears to be mediated by an increase in NOX2 expression.     

Lipoprotein lipase in rats fed fish oil: Apparent relationship to plasma insulin levels

To investigate the role of lipoprotein lipase (LPL) and hepatic lipase in the triacylglycerol lowering effects of fish oil, rats were fed lard (L), corn oil (CO) or menhaden oil (MO) as the primary fat source in otherwise identical diets. After 2 weeks, soleus muscle LPL differed between groups (MO>CO>L). Hepatic lipase did not differ between CO- and MO-fed rats but was elevated in L-fed rats. Adipose LPL did not differ between diet groups. Total epididymal fat weight was reduced in MO-fed rats. There was a significant positive correlation between adipose tissue weight and plasma free fatty acids. MO-fed rats had lower plasma insulin levels. Insulin was directly correlated with plasma triacylglycerol and glucose, consistent with a hyperinsulinemic, insulin-resistant state in CO-and L-fed rats, and a protective effect with MO feeding. In addition, insulin was directly correlated with adipose LPL. A negative relationship between soleus muscle LPL and insulin approached significance. Soleus muscle LPL was significantly inversely correlated with triacylglycerol. The data indicate that increased skeletal muscle LPL, in response to MO or a MO-induced decrease in insulin, may contribute to the triacylglycerol-lowering effects of fish oil. Decreased fat weight and adipose LPL and increased soleus muscle LPL and decreased plasma triacylglycerol suggest a shift from fat deposition to oxidation with MO feeding. The lack of response of hepatic lipase to MO feeding suggests that this enzyme does not contribute to the fish oil-stimulated lowering of plasma triacyglycerolvia hepatic reuptake of very low density lipoproteins or other triacylglycerol-rich lipoproteins.     

哺乳期大鼠大量摄入酒精对子代成年期胰岛素敏感性的影响

目的观察哺乳期大鼠大量摄入酒精对子代成年期胰岛素敏感性的影响。方法将Wistar雌鼠哺乳期酒精灌胃4g/kg·d,对照组给予等容积蒸馏水。子代出生后,每周测量体重1次,断乳后1周开始测量进食量;对第16周龄子代雄性鼠进行静脉葡萄糖耐量试验;Western blot检测骨骼肌细胞膜葡萄糖转运蛋白4(GLUT4)含量;酶学法测定血浆和骨骼肌甘油三酯(TG)含量。结果哺乳期大鼠摄入酒精对后代生长发育无明显影响。与对照组比较,哺乳期大鼠摄入酒精的子代鼠16周龄时,葡萄糖耐量降低,血糖曲线下面积增加,胰岛素敏感指数(SI)、处置指数(DI)及葡萄糖耐量指数(KG)均明显降低,但葡萄糖效应指数(SG)与对照组比较,差异无显著意义,葡萄糖刺激的胰岛素快速反应分泌总量差异亦无显著意义;哺乳期大鼠摄入酒精的子代鼠,骨骼肌细胞膜GLUT4含量、血浆及骨骼肌TG含量与对照组比较,差异均无显著意义。结论哺乳期大鼠大量摄入酒精可引起后代成年期发生胰岛素抵抗,其发生机制与骨骼肌GLUT4转位及TG含量无关。     

Hepatic and Adipose Tissue Depot-Specific Changes in Lipid Metabolism in Late-Onset Obese (LOB) Rats

Transgenic Late-onset OBesity (LOB) rats slowly develop a male-specific, autosomal dominant, obesity phenotype with a specific increase in peri-renal white adipose tissue (WAT) depot and preserved insulin sensitivity (Bains et al. in Endocrinology 145:2666–2679, 2004). To better understand the remarkable phenotype of these rats, the lipid metabolism was investigated in male LOB and non-transgenic (NT) littermates. Total plasma cholesterol (C) levels were normal but total plasma triacylglycerol (TAG) (2.8-fold) and hepatic TAG content (25%) was elevated in LOB males. Plasma VLDL-C and VLDL-TAG levels were higher while plasma apoB levels were 60% lower in LOB males. Increased hepatic TAG secretion explained the increased VLDL levels in LOB males. The hepatic gene expression of FAS, SCD-1, mitochondrial (mt)GPAT, and DGAT2 was up-regulated in both old obese and young non-obese LOB rats. Lipoprotein lipase (LPL) activity in heart and epididymal white adipose tissue (WAT) was unchanged, while LPL activity was increased in peri-renal WAT (30%) and decreased in soleus muscle (40%). Moreover, FAS, SCD-1 and DGAT2 gene expression was increased in peri-renal, but not in epididymal WAT. Basal lipolysis was reduced or unchanged and β-adrenergic stimulated lipolysis was reduced in WAT from both old obese and young non-obese LOB rats. To summarize, the obese phenotype of LOB male rats is associated with increased hepatic TAG production and secretion, a shift in LPL activity from skeletal muscle to WAT, reduced lipolytic response in WAT depots and a specific increase in expression of genes responsible for fatty acid and TAG synthesis in the peri-renal depot. F. Frick and R. Hume contributed equally to this work.     

Activation of the NLRP3 Inflammasome Increases the IL-1β Level and Decreases GLUT4 Translocation in Skeletal Muscle during Insulin Resistance

Low-grade chronic inflammation plays a pivotal role in the pathogenesis of insulin resistance (IR), and skeletal muscle has a central role in this condition. NLRP3 inflammasome activation pathways promote low-grade chronic inflammation in several tissues. However, a direct link between IR and NLRP3 inflammasome activation in skeletal muscle has not been reported. Here, we evaluated the NLRP3 inflammasome components and their role in GLUT4 translocation impairment in skeletal muscle during IR. Male C57BL/6J mice were fed with a normal control diet (NCD) or high-fat diet (HFD) for 8 weeks. The protein levels of NLRP3, ASC, caspase-1, gasdermin-D (GSDMD), and interleukin (IL)-1β were measured in both homogenized and isolated fibers from the flexor digitorum brevis (FDB) or soleus muscle. GLUT4 translocation was determined through GLUT4myc-eGFP electroporation of the FBD muscle. Our results, obtained using immunofluorescence, showed that adult skeletal muscle expresses the inflammasome components. In the FDB and soleus muscles, homogenates from HFD-fed mice, we found increased protein levels of NLRP3 and ASC, higher activation of caspase-1, and elevated IL-1β in its mature form, compared to NCD-fed mice. Moreover, GSDMD, a protein that mediates IL-1β secretion, was found to be increased in HFD-fed-mice muscles. Interestingly, MCC950, a specific pharmacological NLRP3 inflammasome inhibitor, promoted GLUT4 translocation in fibers isolated from the FDB muscle of NCD- and HFD-fed mice. In conclusion, we found increased NLRP3 inflammasome components in adult skeletal muscle of obese insulin-resistant animals, which might contribute to the low-grade chronic metabolic inflammation of skeletal muscle and IR development.     

Oleoyl-Estrone Treatment to Late Pregnant and Mid-Lactating Rats Affects the Expression of Lipid Metabolism Genes

The purpose of this study was to determine whether OE treatment affects the expression of genes related to lipid metabolism under two physiological conditions: late pregnancy and mid-lactation, both characterized by lipid mobilization. Samples of periovarian and retroperitoneal adipose tissue from 21-day pregnant or 15-day lactating dams were used. The expression of LPL, FATP1, FABP4, HSL, ACC1, FAS, PEPCK, GLUT4, PDK4, SREBP1c, adiponutrin and leptin, were compared with their expression in virgin rats. In pregnant rats, FABP4, HSL, PEPCK and PDK4 were over expressed in the periovarian site compared to virgin rats, whereas adiponutrin, FAS, GLUT4 and SREBP1c were underexpressed; the retroperitoneal fat depot showed a similar pattern but ACC1 and leptin were also underexpressed. OE treatment caused a generalized decrease in gene expression in both adipose depots. In lactating dams, the gene expression profile at the periovarian depot was similar to that observed in pregnant rats. OE treatment mimicked the trend observed in pregnant rats, although the intensity of the gene expression changes was lower. After OE treatment, the retroperitoneal adipose depot showed a completely different pattern since the values were close to those of virgin rats. These results corroborate that OE effects in adipose tissue, lowering lipids and depressing their metabolism, already described under other physiological situations, can be also found in late pregnancy and lactation.     

Dietary Monounsaturated Fatty Acids but not Saturated Fatty Acids Preserve the Insulin Signaling Pathway via IRS-1/PI3K in Rat Skeletal Muscle

Saturated fatty acids (SFA) and monounsaturated fatty acids (MUFA) show different effects on the development of insulin resistance. In this study, we compared the effect of dietary SFA and MUFA on the insulin signaling pathway in the skeletal muscle of a type 2 diabetic animal model. Twenty-nine-week-old male Otsuka Long-Evans Tokushima fatty (OLETF) rats were randomly divided into three groups and fed one of the following diets for 3 weeks; a normal chow diet, an SFA (lard oil) enriched or a MUFA (olive oil) enriched high-fat diet. The vastus lateralis muscle was used for analyses. Insulin tolerance test showed improved insulin sensitivity in rats fed the MUFA diet, as compared to those fed the SFA diet (p < 0.001). The SFA diet reduced IRS-1 expression and phosphorylated PI3K levels in skeletal muscle, as compared with a chow diet (p < 0.001, respectively). On the contrary, muscle IRS-2 expression and phosphorylated ERK1/2 was significantly increased in rats fed the SFA diet (p < 0.001, respectively). Membrane translocation of glucose transporter type 4 decreased in the skeletal muscle of rats fed the SFA diet, as compared to those fed a chow diet (p < 0.001). These changes in insulin signaling pathway in skeletal muscle were not observed in rats fed the MUFA diet. In conclusion, the beneficial effect of dietary MUFA on insulin sensitivity is associated with a conserved IRS-1/PI3K insulin signaling pathway which was altered by dietary SFA.     

Olive Leaf Polyphenols (OLPs) Stimulate GLUT4 Expression and Translocation in the Skeletal Muscle of Diabetic Rats

Skeletal muscles are high-insulin tissues responsible for disposing of glucose via the highly regulated process of facilitated glucose transporter 4 (GLUT4). Impaired insulin action in diabetes, as well as disorders of GLUT4 vesicle trafficking in the muscle, are involved in defects in insulin-stimulated GLUT4 translocation. Since the Rab GTPases are the main regulators of vesicular membrane transport in exo- and endo-cytosis, in the present work, we studied the effect of olive leaf polyphenols (OLPs) on Rab8A, Rab13, and Rab14 proteins of the rat soleus muscle in a model of streptozotocin (SZT)-induced diabetes (DM) in a dose-dependent manner. Glucose, cholesterol, and triglyceride levels were determined in the blood, morphological changes of the muscle tissue were captured by hematoxylin and eosin histological staining, and expression of GLUT4, Rab8A, Rab13, and Rab14 proteins were analyzed in the rat soleus muscle by the immunofluorescence staining and immunoblotting. OLPs significantly reduced blood glucose level in all treated groups. Furthermore, significantly reduced blood triglycerides were found in the groups with the lowest and highest OLPs treatment. The dynamics of activation of Rab8A, Rab13, and Rab14 was OLPs dose-dependent and more effective at higher OLP doses. Thus, these results indicate a beneficial role of phenolic compounds from the olive leaf in the regulation of glucose homeostasis in the skeletal muscle.     

Increased Insulin Sensitivity and Distorted Mitochondrial Adaptations during Muscle Unloading

We aimed to further investigate mitochondrial adaptations to muscle disuse and the consequent metabolic disorders. Male rats were submitted to hindlimb unloading (HU) for three weeks. Interestingly, HU increased insulin sensitivity index (ISI) and decreased blood level of triglyceride and insulin. In skeletal muscle, HU decreased expression of pyruvate dehydrogenase kinase 4 (PDK4) and its protein level in mitochondria. HU decreased mtDNA content and mitochondrial biogenesis biomarkers. Dynamin-related protein (Drp1) in mitochondria and Mfn2 mRNA level were decreased significantly by HU. Our findings provide more extensive insight into mitochondrial adaptations to muscle disuse, involving the shift of fuel utilization towards glucose, the decreased mitochondrial biogenesis and the distorted mitochondrial dynamics.     

Comparing the effects of palmitate,insulin, and palmitate-insulin co-treatment on myotube metabolism and insulin resistance

Previous studies have shown various metabolic stressors such as saturated fatty acids (SFA) and excess insulin promote insulin resistance in metabolically meaningful cell types (such as skeletal muscle). Additionally, these stressors have been linked with suppressed mitochondrial metabolism, which is also a common characteristic of skeletal muscle of diabetics. This study characterized the individual and combined effects of excess lipid and excess insulin on myotube metabolism and related metabolic gene and protein expression. C2C12 myotubes were treated with either 500 μM palmitate (PAM), 100 nM insulin (IR), or both (PAM-IR). qRT-PCR and western blot were used to measure metabolic gene and protein expression, respectively. Oxygen consumption was used to measure mitochondrial metabolism. Glycolytic metabolism and insulin-mediated glucose uptake were measured via extracellular acidification rate. Cellular lipid and mitochondrial content were measured using Nile Red and NAO staining, respectively. IR and PAM-IR treatments led to reductions in p-Akt expression. IR treatment reduced insulin mediated glucose metabolism while PAM and PAM-IR treatment showed increases with concurrent reductions in mitochondrial metabolism. All three treatments showed suppression in mitochondrial metabolism. PAM and PAM-IR also showed increases in glycolytic metabolism. While PAM and PAM-IR significantly increased lipid content, expression of inflammatory and lipogenic proteins were unaltered. Lastly, PAM-IR reduced BCAT2 protein expression, a regulator of BCAA metabolism. Both stressors independently reduced insulin signaling, mitochondrial function, and cell metabolism, however, only PAM-IR co-treatment significantly reduced the expression of regulators of metabolism not seen with individual stressors, suggesting an additive effect of stressors on metabolic programming.