Activation Of Insulin Signaling Pathway Research Articles

Advances in Tea and its Bioactive Components Against Diabetic Nephropathy

Diabetic nephropathy has become a serious and growing public health problem. Experimental studies have shown that tea is protective against diabetic nephropathy. Possible mechanisms include enhanced insulin action, improved insulin resistance, activation of insulin signaling pathways, protection of islet β-cells, free radical scavenging, and reduction of inflammation. In addition, clinical trials have confirmed that tea intervention is effective in patients with diabetic nephropathy. Thus, to highlight the importance of tea in the prevention and treatment of diabetic nephropathy, this review summarizes and discusses the effect of tea on diabetic nephropathy based on the results of experimental and clinical studies and pays special attention to its mechanism of action.

The capsaicinoid nonivamide suppresses the inflammatory response and attenuates the progression of steatosis in a NAFLD-rat model.

Nonalcoholic fatty liver disease (NAFLD) is relatively associated with comorbidities in obesity and metabolic inflammation. Low-grade inflammation following the high-fat diet (HFD)-induced NAFLD can promote the development of nonalcoholic steatohepatitis (NASH) through particularly liver-resident immune cell recruitment and hepatic nuclear factor kappa B (NF-κB) pathway. Therefore, inflammatory intervention may contribute to NASH reduction. Pelargonic acid vanillylamide (PAVA) or nonivamide is one of the pungent capsaicinoids of Capsicum species and has been found in chili peppers. Our previous study demonstrated that PAVA improved hepatic function, decreased oxidative stress and reduced apoptotic cell death but the insight role of PAVA on NAFLD is still unclear. Thus, this study aimed to investigate the underlying anti-inflammatory mechanism of PAVA in an NAFLD-rat model. Male Sprague Dawley rats were fed with normal diet or HFD for 16 weeks. Then high-fat rats were given vehicle or PAVA (1 mg/kg/day) for another 4 weeks. We found that PAVA alleviated hepatic inflammation associated with the reducing toll-like receptor 4/NF-κB pathway, showing significantly lower recruitment of cluster of differentiation 44. PAVA also maintained activity of insulin signaling pathway, and attenuated NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome formation. NAFLD progresses to NASH through transforming growth factor (TGF-β1), and also recovery to simple stage followed by PAVA suppresses pro-inflammatory cytokines such as tumor necrosis factor-α, interleukin-1β, interleukin-6, and Janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) pathway. Therefore, our findings suggest that PAVA provides a novel therapeutic approach for NAFLD and slows the progression to NASH.

The identities of insulin signaling pathway are affected by overexpression of Tau and its phosphorylation form.

Hyperphosphorylated Tau formed neurofibrillary tangles was one of the major neuropathological hallmarks of Alzheimer's disease (AD). Dysfunctional insulin signaling in brain is involved in AD. However, the effect of Tau pathology on brain insulin resistance remains unclear. This study explored the effects of overexpressing wild-type Tau (WTau) or Tau with pseudo-phosphorylation at AT8 residues (PTau) on the insulin signaling pathway (ISP). 293T cells or SY5Y cells overexpressing WTau or PTau were treated with or without insulin. The elements in ISP or the regulators of IPS were analyzed by immunoblotting, immunofluorescent staining and co-immunoprecipitation. Akt inhibitor MK2206 was used for evaluating the insulin signaling to downstream of mTOR in Tau overexpressing cells. The effects of anti-aging drug lonafarnib on ISP in WTau or PTau cells were also analyzed with immunoblotting. Considering lonafarnib is an inhibitor of FTase, the states of Rhes, one of FTase substrate in WTau or PTau cells were analyzed by drug affinity responsive target stability (DARTS) assay and the cellular thermal shift assay (CETSA). WTau or PTau overexpression in cells upregulated basal activity of elements in ISP in general. However, overexpression of WTau or PTau suppressed the ISP signaling transmission responses induced by insulin simulation, appearing relative higher response of IRS-1 phosphorylation at tyrosine 612 (IRS-1 p612) in upstream IPS, but a lower phosphorylation response of downstream IPS including mTOR, and its targets 4EPB1 and S6. This dysregulation of insulin evoked signaling transmission was more obvious in PTau cells. Suppressing Akt with MK2206 could compromise the levels of p-S6 and p-mTOR in WTau or PTau cells. Moreover, the changes of phosphatases detected in WTau and PTau cells may be related to ISP dysfunction. In addition, the effects of lonafarnib on the ISP in SY5Y cells with WTau and PTau overexpression were tested, which showed that lonafarnib treatment resulted in reducing the active levels of ISP elements in PTau cells but not in WTau cells. The differential effects are probably due to Tau phosphorylation modulating lonafarnib-induced alterations in Rhes, as revealed by DARTS assay. Overexpression of Tau or Tau with pseudo-phosphorylation at AT8 residues could cause an upregulation of the basal/tonic ISP, but a suppression of insulin induced the phasic activation of ISP. This dysfunction of ISP was more obvious in cells overexpressing pseudo-phosphorylated Tau. These results implied that the dysfunction of ISP caused by Tau overexpression might impair the physiological fluctuation of neuronal functions in AD. The different effects of lonafarnib on ISP between WTau and PTau cells, indicating that Tau phosphorylation mediates an additional effect on ISP. This study provided a potential linkage of abnormal expression and phosphorylation of Tau to the ISP dysfunction in AD.

Comparative single-cell transcriptomic analysis reveals differences in signaling pathways in gonadal primordial germ cells between chicken (Gallus gallus) and zebra finch (Taeniopygia guttata).

Primordial germ cells (PGCs) have been used in avian genetic resource conservation and transgenic animal production. Despite their potential applications to numerous avian taxa facing extinction due to habitat loss and degradation, research has largely focused on poultry, such as chickens, in part owing to the difficulty in obtaining intact PGCs from other species. Recently, phenotypic differences between PGCs of chicken and zebra finch, a wild bird with vocal learning, in early embryonic development have been reported. In this study, we used advanced single-cell RNA sequencing (scRNA-seq) technology to evaluate zebra finch and chicken PGCs and surrounding cells, and to identify species-specific characteristics. We constructed single-cell transcriptome landscapes of chicken gonadal PGCs for a comparison with previously reported scRNA-seq data for zebra finch. We identified interspecific differences in several signaling pathways in gonadal PGCs and somatic cells. In particular, NODAL and insulin signaling pathway activity levels were higher in zebra finch than in chickens, whereas activity levels of the downstream FGF signaling pathway, involved in the proliferation of chicken PGCs, were higher in chickens. This study is the first cross-species single-cell transcriptomic analysis targeting birds, revealing differences in germ cell development between phylogenetically distant Galliformes and Passeriformes. Our results provide a basis for understanding the reproductive physiology of avian germ cells and for utilizing PGCs in the restoration of endangered birds and the production of transgenic birds.

Transgenerational effects of zinc, selenium and chromium supplementation on glucose homeostasis in female offspring of gestational diabetes rats

Clinical studies have demonstrated that maternal gestational diabetes mellitus (GDM) increases the offspring's risk of developing glucose intolerance. Our previous study reported that co-supplementation with zinc, selenium, and chromium improved insulin resistance in diet-induced GDM rats. Here, Transgenerational effects of supplementation with zinc (10 mg/kg.bw), selenium (20 μg/kg.bw), and chromium (20 μg/kg.bw) in F1 female offspring of both zinc, selenium and chromium (ZnSeCr)-treated, and untreated GDM rats daily by gavage from weaning to the postpartum were investigated in the present study. Glucose homeostasis in the F1 female offspring of GDM at different stages were evaluated. Maternal GDM did increase the birth mass of newborn F1 female offspring, as well as the serum glucose and insulin levels. Zinc, selenium and chromium supplementation attenuated the GDM-induced mass gain, increased serum glucose and insulin levels in the female neonates. The high fat and sucrose (HFS) diet-fed GDM-F1 offspring developed GDM, with glucose intolerance, hyperglycemia and insulin resistance during pregnancy. Moreover, endoplasmic reticulum (ER) stress-related protein levels were increased and the activation of insulin signaling pathways were reduced in the liver of HFS-fed GDM-F1 offspring. Whereas glucose homeostasis in parallel with insulin sensitivity was normalized in the female offspring of GDM by supplementation both F0 dams and F1 offspring with zinc, selenium and chromium, not in those either F0 or F1 elements supplemented offspring. Therefore, we speculate that zinc, selenium and chromium supplementation may have a potential beneficial transgenerational effect on the glucose homeostasis in the female offspring of GDM.

Circulating small extracellular vesicles from patients with periodontitis contribute to development of insulin resistance.

Epidemiological studies have identified the role of periodontitis in the pathogenesis of type 2 diabetes, but the underlying mechanism is poorly understood. It is well-known that small extracellular vesicles are lipid bilayer vesicles derived from cells with a diameter around 30 to 200nm. The purpose of this study was to investigate whether periodontitis induced or exacerbated insulin resistance via circulating small extracellular vesicles. Plasma small extracellular vesicles from control and periodontitis rats were intravenously injected into type 2 diabetic rats. Insulin tolerance tests, glucose tolerance tests, and the activation of the insulin signaling pathway were measured to detect the effect of the plasma small extracellular vesicles on insulin sensitivity. In addition, circulating small extracellular vesicles from patients with periodontitis with or without diabetes were isolated and co-cultured with HepG2 cells. The ability of glucose uptake was assessed using the fluorescence of 2-NBDG via flow cytometry. The activation of insulin signaling pathway was examined via Western blotting. Real time quantitative polymerase chain reaction (RT-qPCR) was used to detect the expression of enzyme related to glycolysis and gluconeogenesis. Small extracellular vesicles derived from the plasma of periodontitis rats further impaired glucose tolerance and insulin tolerance in diabetic rats and significantly reduced the activation of the insulin signaling pathway in liver tissues, as evidenced by the decreased levels of p-AKT and p-GSK3β and the reduced hepatic glycogen content. For small extracellular vesicles isolated from human plasma, the concentration of small extracellular vesicles in patients with type 2 diabetes combined with periodontitis was higher than that of the healthy control and periodontitis alone. Moreover, circulating small extracellular vesicles from patients with periodontitis significantly inhibited the glucose uptake capacity and inhibited insulin signaling of HepG2 cells. Periodontitis acted as a contributing factor to exacerbate insulin resistance of type 2 diabetic rats. Plasma small extracellular vesicles played a critical role in periodontitis aggravating insulin resistance.

Insulin gene expression and functional activity of insulin signaling pathway in Alzheimer's disease

Aim. To study the insulin (INS) gene expression, insulin and lactate levels, expression of Fe65 adapter protein, and level of oxidative DNA damage marker γH2AX in different brain areas in the experimental model of Alzheimer's disease.Materials and Methods. Male, 4-month-old C57BL/6 mice received either intrahippocampal injection of β-amyloid (C57BL/6 + Aβ 1-42) or phosphate-buffered saline (C57BL/6 + PBS). Insulin (INS) gene expression in the hippocampus and amygdala was assessed by means of reverse transcription-polymerase chain reaction. Levels of lactate and insulin in different brain areas were measured by enzyme-linked immunosorbent assay. Expression of Fe65 adapter protein and γH2AX in the hippocampus was studied by immunofluorescence staining followed by confocal microscopy.Results. We found an overexpression of the INS gene in the hippocampus and amygdala, an increase in lactate level in the hippocampus, and slightly increased insulin level in the amygdala of mice with Alzheimer's disease as compared with the control group. Neurodegeneration was accompanied by an elevated endothelial expression of Fe65 adapter protein (p= 0.04) and γH2AX in hippocampal neurons (p = 0.04).Conclusion. Alzheimer's disease neurodegeneration is accompanied by a disrupted insulin signaling and impaired glucose metabolism in the hippocampus and amygdala. This further leads to a neuronal accumulation of γH2AX and impaired amyloid precursor protein proteolysis because of insulin inability to inhibit its interaction with the Fe65 adapter protein and to prevent formation and deposition of β-amyloid.

Opposite Effects of Chronic Central Leptin Infusion on Activation of Insulin Signaling Pathways in Adipose Tissue and Liver Are Related to Changes in the Inflammatory Environment.

Leptin modulates insulin signaling and this involves the Akt pathway, which is influenced by changes in the inflammatory environment and with leptin regulating cytokine synthesis. We evaluated the association between activation of the insulin-signaling pathway and alterations in pro- and anti-inflammatory cytokine levels in inguinal fat and liver of chronic central leptin infused (L), pair-fed (PF), and control rats. Signal transducer and activator of transcription 3 (STAT3) phosphorylation was increased in inguinal fat and reduced in liver of L rats. Phosphorylation of c-Jun N-terminal kinase (JNK) and nuclear factor kappa B (NFkB) was increased in inguinal fat of L rats, together with a pro-inflammatory cytokine profile, while in the liver activation of JNK and NFkB were reduced and an anti-inflammatory pattern was found. Phosphorylation of the insulin receptor, Akt and mechanistic target of rapamycin was decreased in inguinal fat and increased in liver of L rats. There was a direct relationship between pSTAT3 and JNK and a negative correlation of Akt with pSTAT3 and JNK in both tissues. These results indicate that the effects of chronically increased leptin on insulin-related signaling are tissue-specific and suggest that inflammation plays a relevant role in the crosstalk between leptin and insulin signaling.

Effective amelioration of hepatic inflammation and insulin response in high fat diet-fed rats via regulating AKT/mTOR signaling: Role of Lepidium sativum seed extracts

Ethnopharmacological relevanceObesity-induced insulin resistance and chronic inflammation appears to be the most frequent cause of diabetes and its related metabolic complications; in this way a new therapeutic approaches are needed to prevent the chronic obesity and insulin resistance. Lepidium sativum has been extensively used in traditional alternative medicine for cough, skin disease, liver disorder, diuretic, gastrointestinal problems, hair loss treatment, milk secretion during lactation as well as antioxidant, antihypertensive, anti-inflammatory, and antidiabetic activities. The hypoglycemic and hypolipidemic effect of Lepidium sativum have been observed by previous studies, but the underlying molecular mechanisms are unclear. Aim of the studyIn this study, we investigated the beneficial effect of Lepidium sativum ethanol and aqueous seed extracts on obesity, oxidative, inflammatory, and insulin sensitivity changes in the liver tissue of high fat diet (HFD)-fed rats. The bioactive constituents responsible for these activities have been identified for both extracts using HPLC and GC-MS. Materials and methodsRats were fed HFD for 10 weeks. The obese rats were treated orally with the Lepidium sativum ethanol extracts (LSEE) at dose 200 and 400 mg/kg body weight (BW) and Lepidium sativum aqueous extracts (LSAE) at dose 200 mg/kg BW daily for 8 weeks. ResultsThe findings of the present study pointed out a significant increase in the hepatic transaminases, lipid profile, leptin, and hepatic oxidative stress with decreased antioxidant capacity of HFD-fed rats. Consistent with this depiction; we determined the up-regulation of liver inflammatory markers with a significant down-regulation of insulin signaling components phospho-insulin receptor (p-IR), p-AKT, p-mammalian target of rapamycin (p-mTOR), and p-p70S6K after consumption of HFD for 10 weeks that indicates a deterioration of insulin sensitivity. Interestingly, the phytochemical screening of LSEE and LSAE exhibited positive results for phenolic, flavonoid, lipid, and some bioactive components as well as the in vitro antioxidant activity of both extracts clearly demonstrated their high antioxidant activities. Notably, LSEE and LSAE displayed a wide range of biological features including anti-obesity, anti-inflammatory, and antioxidant properties. Both extracts significantly decreased high glucose, leptin, lipid profile, liver enzymes levels, and body weight. We also found that LSEE and LSAE significantly alleviated lipid peroxidation and restored the antioxidant enzymes to normal levels. In parallel, the intracellular phosphorylation of classical markers of insulin signaling cascade p-IR/p-AKT/p-mTOR/p-p70S6K was up-regulated in the hepatic tissues of LSEE and LSAE-treated groups. ConclusionThis study provides evidence that LSEE and LSAE might be one promising dietary supplementation that could safely and effectively prevent the early metabolic alterations and weight gain caused by HFD further regulate the activation of insulin signaling pathway beside their powerful antioxidant and low-toxicity properties.

468-P: PDK1 Is a Key Regulator of Podocyte Viability and Maintenance of Glomerular Function

In diabetic states, altered activity of insulin signaling pathway is likely to play key roles in insulin resistance and in kidney dysfunction. Since the 3-phosphoinositide-dependent protein kinase 1 (PDK1) serves as a major signaling molecule in the regulation of cellular responses in multiple organs by mediating the phosphoinositide 3-kinase signaling pathway, we tested the hypothesis that podocyte-specific deletion of PDK1 leads to podocyte injury and proteinuric kidney disease. PDK1 was selectively silenced in podocytes using Cre-mediated recombination controlled by the podocin promoter. Mice with a podocyte-specific deletion of PDK1 (PDK1 KO) developed proteinuria, and died due to end-stage renal disease by 5 weeks after birth. Histologically, features characteristic of glomerulosclerosis, including podocyte foot process effacement, mesangial sclerosis, and protein casts, were observed in PDK1 KO at 3 weeks of age. We also examined podocyte distribution in glomeruli using the podocyte markers synaptopodin and WT-1, and found that their expressions were lost in some glomeruli of PDK1 KO at 3 weeks of age although their expression patterns at 1 weeks of age PDK1 KO were almost comparable with control mice. We next examined a possible role of forkhead transcription factor, forkhead box O1 (FoxO1) in linking PDK1 of podocytes to kidney function. For this, we generated podocyte-specific PDK1 and FoxO1 double knockout mice (DKO) by introducing Foxo1-flox gene into PDK1 KO mice. DKO exhibited a reduced level of proteinuria compared with age-matched PDK1 KO, and the maximal survival time of DKO had extended to 6.6 weeks after birth. Taken together, PDK1-FoxO1 pathway may be one of the critical signal transduction pathways that links podocyte injury and kidney dysfunction in diabetic states. Disclosure W. Iwata: None. H. Unoki-Kubota: None. H. Kato: None. M. Matsumoto: None. M. Nangaku: Consultant; Self; Akebia Therapeutics, Astellas Pharma Inc., Bayer AG, Boehringer Ingelheim International GmbH, Daiichi Sankyo, GlaxoSmithKline plc., Japan Tobacco Inc., Kyowa Hakko Kirin Co., Ltd., Mitsubishi Tanabe Pharma Corporation, Novo Nordisk Inc., Otsuka Pharmaceutical Co., Ltd., Scohia Pharma Inc. Research Support; Self; Takeda Pharmaceutical Company Limited. Speaker’s Bureau; Self; AstraZeneca, Torii Pharmaceutical Co. Ltd. Y. Kaburagi: None.

Intestine-specific activity of insulin signaling pathway in response to microgravity stress in Caenorhabditis elegans

Insulin signaling pathway is conserved and important during the control of biological processes in organisms. In this study, using the simulated microgravity system in Caenorhabditis elegans, we examined the response of insulin signaling pathway to simulated microgravity. After simulated microgravity treatment for 24-h, we observed the significant decrease in expressions of daf-2 encoding an insulin receptor and two genes (age-1 and akt-1) encoding kinase proteins, and increase in expression of daf-16 encoding a FOXO transcriptional factor. Simulated microgravity also induced an obvious translocation and accumulation of DAF-16::GFP signals in the nucleus. Mutation of daf-2, age-1, or akt-1 caused the resistance to toxicity of simulated microgravity, and daf-16 mutation caused the susceptibility to toxicity of simulated microgravity, indicating the protective response of insulin signaling to simulated microgravity. RNA interference (RNAi) knockdown of daf-16 could suppress the resistance of daf-2, age-1, or akt-1 mutant nematodes to toxicity of simulated microgravity, implying the involvement of signaling cascade of DAF-2-AGE-1-AKT-1-DAF-16 in regulating the response to simulated microgravity. Tissue-specific activity analysis demonstrated that the insulin signaling pathway only acted in the intestine to regulate the response to simulated microgravity. Moreover, in the intestine, insulin signaling functioned synergistically with p38 mitogen-activated protein kinase (MAPK) signaling to regulate the response to simulated microgravity. Therefore, our results suggest the crucial role of intestinal insulin signaling pathway in response to simulate microgravity in nematodes.

Positive Feedback Regulation Effect Exits Between Activation of Insulin Signaling Pathway and Apolipoprotein M Expression

Positive Feedback Regulation Effect Exits Between Activation of Insulin Signaling Pathway and Apolipoprotein M Expression

GLP‑1 improves palmitate‑induced insulin resistance in human skeletal muscle via SIRT1 activity.

The present study investigated whether glucagon like peptide‑1 (GLP‑1) improves glucose uptake through glucose transporter type 4 (GLUT4), mediated by the activation of sirtuin 1 (SIRT1), in skeletal muscle cells with palmitate induced‑insulin resistance. The levels of glucose uptake, GLUT4, protein kinase A (PKA), and cyclic adenosine monophosphate (cAMP) were determined in human skeletal muscle myotubes (HSMMs) exposed to palmitate and GLP‑1. Then, to determine whether PKA/cAMP were downstream signals of GLP‑1, a PKA inhibitor was used. To determine whether SIRT‑1 contributes to GLP‑1 action in HSMMs with palmitate‑induced insulin resistance, the levels of peroxisome proliferator‑activated receptor γ coactivator 1α (PGC1α) deacetylation and SIRT‑1 activity were assessed using a SIRT1 inhibitor and small interfering RNA (siRNA). The phosphorylation levels of protein kinase B (Akt) and insulin receptor substrate 1 (IRS‑1) as insulin signaling pathways, were assessed in GLP‑1‑treated HSMMs exposed to palmitate. The influence of SIRT1 on the GLP‑1‑induced activation of insulin signaling pathway was determined using a SIRT1 inhibitor. GLP‑1 restored the palmitate‑induced reductions in the levels of glucose uptake, GLUT4 mRNA, GLUT4 promoter activity, and GLUT4 protein in HSMMs. PKA and cAMP, as GLP‑1 downstream signals, played a role in this process. GLP‑1 increased the deacetylation levels of PGC1α, and stimulated SIRT1 in HSMMs. Moreover, the SIRT1 inhibitor and siRNA of SIRT1 suppressed the effect of GLP‑1 on GLUT4 expression in HSMMs exposed to palmitate. The SIRT1 inhibitor also prevented the GLP‑1‑induced phosphorylation of IRS‑1 and Akt in palmitate‑treated HSMMs. The present findings suggest that in palmitate‑induced insulin‑resistant HSMM, GLP‑1 activates SIRT1 through the PKA/cAMP pathway, which in turn enhances glucose uptake through GLUT4 and the insulin signaling pathway.

Protein Kinase C Epsilon Deletion in Adipose Tissue, but Not in Liver, Improves Glucose Tolerance

Protein kinase C epsilon (PKCɛ) activation in the liver is proposed to inhibit insulin action through phosphorylation of the insulin receptor. Here, however, we demonstrated that global, but not liver-specific, deletion of PKCɛ in mice protected against diet-induced glucose intolerance and insulin resistance. Furthermore, PKCɛ-dependent alterations in insulin receptor phosphorylation were not detected. Adipose-tissue-specific knockout mice did exhibit improved glucose tolerance, but phosphoproteomics revealed no PKCɛ-dependent effect on the activation of insulin signaling pathways. Altered phosphorylation of adipocyte proteins associated with cell junctions and endosomes was associated with changes in hepatic expression of several genes linked to glucose homeostasis and lipid metabolism. The primary effect of PKCɛ on glucose homeostasis is, therefore, not exerted directly in the liver as currently posited, and PKCɛ activation in this tissue should be interpreted with caution. However, PKCɛ activity in adipose tissue modulates glucose tolerance and is involved in crosstalk with the liver.

Liver-Specific Overexpression of Gamma-Glutamyltransferase Ameliorates Insulin Sensitivity of Male C57BL/6 Mice.

In the current study, we developed a liver-specific GGT-overexpressing mice model by rapid injection pLIVE-GGT vector through tail vein and investigated the effects of GGT elevation on glucose metabolism and insulin sensitivity. The serum GGT activity was significantly increased after 7 days of pLIVE-GGT1 vector delivery and lasted for about 3 weeks. GGT overexpression reduced the levels of GSSG and GSH in the liver and serum and had no effects on total antioxidative capacity in the liver, kidney, and skeletal muscle except for the pancreas. Increased GGT activity had no effect on the glucose tolerance but could facilitate blood glucose lowering after intraperitoneal insulin administration. The results of Western blotting showed that increased GGT activity enhanced insulin-induced AKT phosphorylation at Ser473. Furthermore, GGT inhibitor could attenuate the changes of insulin-induced blood glucose uptake and AKT phosphorylation in the liver. In summary, the present study developed a liver-specific GGT-overexpressing mice model and found that GGT elevation in short term had no effects on glucose metabolism but could increase insulin sensitivity through enhancing the activity of insulin signaling pathway.

Resistant starch attenuates impaired lipid biosynthesis induced by dietary oxidized oil via activation of insulin signaling pathways

The current study found that deep-frying process led to an increased content of oxidized triacylglycerols in canola oil, 3.5 times higher than that of fresh canola oil (not used for frying).

Sustained Action of Ceramide on the Insulin Signaling Pathway in Muscle Cells: IMPLICATION OF THE DOUBLE-STRANDED RNA-ACTIVATED PROTEIN KINASE

In vivo, ectopic accumulation of fatty acids in muscles leads to alterations in insulin signaling at both the IRS1 and Akt steps. However, in vitro treatments with saturated fatty acids or their derivative ceramide demonstrate an effect only at the Akt step. In this study, we adapted our experimental procedures to mimic the in vivo situation and show that the double-stranded RNA-dependent protein kinase (PKR) is involved in the long-term effects of saturated fatty acids on IRS1. C2C12 or human muscle cells were incubated with palmitate or directly with ceramide for short or long periods, and insulin signaling pathway activity was evaluated. PKR involvement was assessed through pharmacological and genetic studies. Short-term treatments of myotubes with palmitate, a ceramide precursor, or directly with ceramide induce an inhibition of Akt, whereas prolonged periods of treatment show an additive inhibition of insulin signaling through increased IRS1 serine 307 phosphorylation. PKR mRNA, protein, and phosphorylation are increased in insulin-resistant muscles. When PKR activity is reduced (siRNA or a pharmacological inhibitor), serine phosphorylation of IRS1 is reduced, and insulin-induced phosphorylation of Akt is improved. Finally, we show that JNK mediates ceramide-activated PKR inhibitory action on IRS1. Together, in the long term, our results show that ceramide acts at two distinct levels of the insulin signaling pathway (IRS1 and Akt). PKR, which is induced by both inflammation signals and ceramide, could play a major role in the development of insulin resistance in muscle cells.

Bilirubin Increases Insulin Sensitivity in Leptin-Receptor Deficient and Diet-Induced Obese Mice Through Suppression of ER Stress and Chronic Inflammation

Obesity-induced endoplasmic reticulum (ER) stress causes chronic inflammation in adipose tissue and steatosis in the liver, and eventually leads to insulin resistance and type 2 diabetes (T2D). The goal of this study was to understand the mechanisms by which administration of bilirubin, a powerful antioxidant, reduces hyperglycemia and ameliorates obesity in leptin-receptor-deficient (db/db) and diet-induced obese (DIO) mouse models. db/db or DIO mice were injected with bilirubin or vehicle ip. Blood glucose and body weight were measured. Activation of insulin-signaling pathways, expression of inflammatory cytokines, and ER stress markers were measured in skeletal muscle, adipose tissue, and liver of mice. Bilirubin administration significantly reduced hyperglycemia and increased insulin sensitivity in db/db mice. Bilirubin treatment increased protein kinase B (PKB/Akt) phosphorylation in skeletal muscle and suppressed expression of ER stress markers, including the 78-kDa glucose-regulated protein (GRP78), CCAAT/enhancer-binding protein (C/EBP) homologous protein, X box binding protein (XBP-1), and activating transcription factor 4 in db/db mice. In DIO mice, bilirubin treatment significantly reduced body weight and increased insulin sensitivity. Moreover, bilirubin suppressed macrophage infiltration and proinflammatory cytokine expression, including TNF-α, IL-1β, and monocyte chemoattractant protein-1, in adipose tissue. In liver and adipose tissue of DIO mice, bilirubin ameliorated hepatic steatosis and reduced expression of GRP78 and C/EBP homologous protein. These results demonstrate that bilirubin administration improves hyperglycemia and obesity by increasing insulin sensitivity in both genetically engineered and DIO mice models. Bilirubin or bilirubin-increasing drugs might be useful as an insulin sensitizer for the treatment of obesity-induced insulin resistance and type 2 diabetes based on its profound anti-ER stress and antiinflammatory properties.

Evidence for a regulatory role of Cullin-RING E3 ubiquitin ligase 7 in insulin signaling

Dysfunctional regulation of signaling pathways downstream of the insulin receptor plays a pivotal role in the pathogenesis of insulin resistance and type 2 diabetes. In this study we report both in vitro and in vivo experimental evidence for a role of Cullin-RING E3 ubiquitin ligase 7 (CRL7) in the regulation of insulin signaling and glucose homeostasis. We show that Cul7−/− mouse embryonic fibroblasts displayed enhanced AKT and Erk MAP kinase phosphorylation upon insulin stimulation. Depletion of CUL7 by RNA interference in C2C12 myotubes led to increased activation of insulin signaling pathways and cellular glucose uptake, as well as a reduced capacity of these cells to execute insulin-induced degradation of insulin receptor substrate 1 (IRS1). In vivo, heterozygosity of either Cul7 or Fbxw8, both key components of CRL7, resulted in elevated PI3 kinase/AKT activation in skeletal muscle tissue upon insulin stimulation when compared to wild-type controls. Finally, Cul7+/− or Fbxw8+/− mice exhibited enhanced insulin sensitivity and plasma glucose clearance. Collectively, our findings point to a yet unrecognized role of CRL7 in insulin-mediated control of glucose homeostasis by restraining PI3 kinase/AKT activities in skeletal muscle cells.

Grape skin extract protects against programmed changes in the adult rat offspring caused by maternal high-fat diet during lactation

Maternal overnutrition during suckling period is associated with increased risk of metabolic disorders in the offspring. We aimed to assess the effect of Vitis vinifera L. grape skin extract (ACH09) on cardiovascular and metabolic disorders in adult male offspring of rats fed a high-fat (HF) diet during lactation. Four groups of female rats were fed: control diet (7% fat), ACH09 (7% fat plus 200 mg kg−1 d−1 ACH09 orally), HF (24% fat), and HF+ACH09 (24% fat plus 200 mg kg−1 d−1 ACH09 orally) during lactation. After weaning, all male offspring were fed a control diet and sacrificed at 90 or 180 days old. Systolic blood pressure was increased in adult offspring of HF-fed dams and ACH09 prevented the hypertension. Increased adiposity, plasma triglyceride, glucose levels and insulin resistance were observed in offspring from both ages, and those changes were reversed by ACH09. Expression of insulin cascade proteins IRS-1, AKT and GLUT4 in the soleus muscle was reduced in the HF group of both ages and increased by ACH09. The plasma oxidative damage assessed by malondialdehyde levels was increased, and nitrite levels decreased in the HF group of both ages, which were reversed by ACH09. In addition, ACH09 restored the decreased plasma and mesenteric arteries antioxidant activities of superoxide dismutase, catalase and glutathione peroxidase in the HF group. In conclusion, the treatment of HF-fed dams during lactation with ACH09 provides protection from later-life hypertension, body weight gain, insulin resistance and oxidative stress. The protective effect ACH09 may involve NO synthesis, antioxidant action and activation of insulin-signaling pathways.